diff --git a/go.mod b/go.mod
index cba505e..8e6266d 100644
--- a/go.mod
+++ b/go.mod
@@ -9,6 +9,8 @@
 	github.com/GoogleCloudPlatform/opentelemetry-operations-go/exporter/metric v0.26.0
 	github.com/GoogleCloudPlatform/opentelemetry-operations-go/exporter/trace v1.0.0
 	github.com/client9/misspell v0.3.4
+	github.com/go-git/go-billy/v5 v5.4.0
+	github.com/go-git/go-git/v5 v5.5.2
 	github.com/google/go-cmp v0.5.9
 	go.opencensus.io v0.24.0
 	go.opentelemetry.io/otel v1.4.0
@@ -29,17 +31,31 @@
 	cloud.google.com/go/monitoring v1.8.0 // indirect
 	cloud.google.com/go/trace v1.4.0 // indirect
 	github.com/BurntSushi/toml v0.3.1 // indirect
+	github.com/Microsoft/go-winio v0.5.2 // indirect
+	github.com/ProtonMail/go-crypto v0.0.0-20221026131551-cf6655e29de4 // indirect
+	github.com/acomagu/bufpipe v1.0.3 // indirect
+	github.com/cloudflare/circl v1.1.0 // indirect
+	github.com/emirpasic/gods v1.18.1 // indirect
+	github.com/go-git/gcfg v1.5.0 // indirect
 	github.com/go-logr/logr v1.2.2 // indirect
 	github.com/go-logr/stdr v1.2.2 // indirect
 	github.com/golang/groupcache v0.0.0-20210331224755-41bb18bfe9da // indirect
 	github.com/golang/protobuf v1.5.2 // indirect
 	github.com/googleapis/enterprise-certificate-proxy v0.2.0 // indirect
 	github.com/googleapis/gax-go/v2 v2.7.0 // indirect
+	github.com/imdario/mergo v0.3.13 // indirect
+	github.com/jbenet/go-context v0.0.0-20150711004518-d14ea06fba99 // indirect
+	github.com/kevinburke/ssh_config v1.2.0 // indirect
+	github.com/pjbgf/sha1cd v0.2.3 // indirect
+	github.com/sergi/go-diff v1.1.0 // indirect
+	github.com/skeema/knownhosts v1.1.0 // indirect
+	github.com/xanzy/ssh-agent v0.3.3 // indirect
 	go.opentelemetry.io/otel/internal/metric v0.27.0 // indirect
 	go.opentelemetry.io/otel/metric v0.27.0 // indirect
 	go.opentelemetry.io/otel/sdk/export/metric v0.26.0 // indirect
 	go.opentelemetry.io/otel/sdk/metric v0.26.0 // indirect
 	go.opentelemetry.io/otel/trace v1.4.0 // indirect
+	golang.org/x/crypto v0.3.0 // indirect
 	golang.org/x/oauth2 v0.0.0-20221014153046-6fdb5e3db783 // indirect
 	golang.org/x/sync v0.1.0 // indirect
 	golang.org/x/sys v0.4.0 // indirect
@@ -49,4 +65,5 @@
 	google.golang.org/genproto v0.0.0-20221201164419-0e50fba7f41c // indirect
 	google.golang.org/grpc v1.50.1 // indirect
 	google.golang.org/protobuf v1.28.1 // indirect
+	gopkg.in/warnings.v0 v0.1.2 // indirect
 )
diff --git a/go.sum b/go.sum
index 7eb757f..200485b 100644
--- a/go.sum
+++ b/go.sum
@@ -75,10 +75,21 @@
 github.com/GoogleCloudPlatform/opentelemetry-operations-go/exporter/metric v0.26.0/go.mod h1:KKljpKOZZUG9ljcNQJgKV88u8CXbXibh6OqM9rMVlIA=
 github.com/GoogleCloudPlatform/opentelemetry-operations-go/exporter/trace v1.0.0 h1:38fNtfhHY6bs22b/D6+hDzO6JR0rDzpGPD36dY2uPL4=
 github.com/GoogleCloudPlatform/opentelemetry-operations-go/exporter/trace v1.0.0/go.mod h1:jE23wM1jvwSKgdGcoOkj5j9n1VWtncW36pL2bK1JU+0=
+github.com/Microsoft/go-winio v0.5.2 h1:a9IhgEQBCUEk6QCdml9CiJGhAws+YwffDHEMp1VMrpA=
+github.com/Microsoft/go-winio v0.5.2/go.mod h1:WpS1mjBmmwHBEWmogvA2mj8546UReBk4v8QkMxJ6pZY=
 github.com/OneOfOne/xxhash v1.2.2/go.mod h1:HSdplMjZKSmBqAxg5vPj2TmRDmfkzw+cTzAElWljhcU=
+github.com/ProtonMail/go-crypto v0.0.0-20221026131551-cf6655e29de4 h1:ra2OtmuW0AE5csawV4YXMNGNQQXvLRps3z2Z59OPO+I=
+github.com/ProtonMail/go-crypto v0.0.0-20221026131551-cf6655e29de4/go.mod h1:UBYPn8k0D56RtnR8RFQMjmh4KrZzWJ5o7Z9SYjossQ8=
+github.com/acomagu/bufpipe v1.0.3 h1:fxAGrHZTgQ9w5QqVItgzwj235/uYZYgbXitB+dLupOk=
+github.com/acomagu/bufpipe v1.0.3/go.mod h1:mxdxdup/WdsKVreO5GpW4+M/1CE2sMG4jeGJ2sYmHc4=
+github.com/anmitsu/go-shlex v0.0.0-20200514113438-38f4b401e2be h1:9AeTilPcZAjCFIImctFaOjnTIavg87rW78vTPkQqLI8=
+github.com/anmitsu/go-shlex v0.0.0-20200514113438-38f4b401e2be/go.mod h1:ySMOLuWl6zY27l47sB3qLNK6tF2fkHG55UZxx8oIVo4=
 github.com/antihax/optional v1.0.0/go.mod h1:uupD/76wgC+ih3iEmQUL+0Ugr19nfwCT1kdvxnR2qWY=
+github.com/armon/go-socks5 v0.0.0-20160902184237-e75332964ef5 h1:0CwZNZbxp69SHPdPJAN/hZIm0C4OItdklCFmMRWYpio=
+github.com/armon/go-socks5 v0.0.0-20160902184237-e75332964ef5/go.mod h1:wHh0iHkYZB8zMSxRWpUBQtwG5a7fFgvEO+odwuTv2gs=
 github.com/benbjohnson/clock v1.3.0 h1:ip6w0uFQkncKQ979AypyG0ER7mqUSBdKLOgAle/AT8A=
 github.com/benbjohnson/clock v1.3.0/go.mod h1:J11/hYXuz8f4ySSvYwY0FKfm+ezbsZBKZxNJlLklBHA=
+github.com/bwesterb/go-ristretto v1.2.0/go.mod h1:fUIoIZaG73pV5biE2Blr2xEzDoMj7NFEuV9ekS419A0=
 github.com/census-instrumentation/opencensus-proto v0.2.1/go.mod h1:f6KPmirojxKA12rnyqOA5BBL4O983OfeGPqjHWSTneU=
 github.com/cespare/xxhash v1.1.0/go.mod h1:XrSqR1VqqWfGrhpAt58auRo0WTKS1nRRg3ghfAqPWnc=
 github.com/cespare/xxhash/v2 v2.1.1/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
@@ -87,6 +98,8 @@
 github.com/chzyer/test v0.0.0-20180213035817-a1ea475d72b1/go.mod h1:Q3SI9o4m/ZMnBNeIyt5eFwwo7qiLfzFZmjNmxjkiQlU=
 github.com/client9/misspell v0.3.4 h1:ta993UF76GwbvJcIo3Y68y/M3WxlpEHPWIGDkJYwzJI=
 github.com/client9/misspell v0.3.4/go.mod h1:qj6jICC3Q7zFZvVWo7KLAzC3yx5G7kyvSDkc90ppPyw=
+github.com/cloudflare/circl v1.1.0 h1:bZgT/A+cikZnKIwn7xL2OBj012Bmvho/o6RpRvv3GKY=
+github.com/cloudflare/circl v1.1.0/go.mod h1:prBCrKB9DV4poKZY1l9zBXg2QJY7mvgRvtMxxK7fi4I=
 github.com/cncf/udpa/go v0.0.0-20191209042840-269d4d468f6f/go.mod h1:M8M6+tZqaGXZJjfX53e64911xZQV5JYwmTeXPW+k8Sc=
 github.com/cncf/udpa/go v0.0.0-20200629203442-efcf912fb354/go.mod h1:WmhPx2Nbnhtbo57+VJT5O0JRkEi1Wbu0z5j0R8u5Hbk=
 github.com/cncf/udpa/go v0.0.0-20201120205902-5459f2c99403/go.mod h1:WmhPx2Nbnhtbo57+VJT5O0JRkEi1Wbu0z5j0R8u5Hbk=
@@ -95,9 +108,12 @@
 github.com/cncf/xds/go v0.0.0-20210805033703-aa0b78936158/go.mod h1:eXthEFrGJvWHgFFCl3hGmgk+/aYT6PnTQLykKQRLhEs=
 github.com/cncf/xds/go v0.0.0-20210922020428-25de7278fc84/go.mod h1:eXthEFrGJvWHgFFCl3hGmgk+/aYT6PnTQLykKQRLhEs=
 github.com/cncf/xds/go v0.0.0-20211011173535-cb28da3451f1/go.mod h1:eXthEFrGJvWHgFFCl3hGmgk+/aYT6PnTQLykKQRLhEs=
+github.com/creack/pty v1.1.9/go.mod h1:oKZEueFk5CKHvIhNR5MUki03XCEU+Q6VDXinZuGJ33E=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/emirpasic/gods v1.18.1 h1:FXtiHYKDGKCW2KzwZKx0iC0PQmdlorYgdFG9jPXJ1Bc=
+github.com/emirpasic/gods v1.18.1/go.mod h1:8tpGGwCnJ5H4r6BWwaV6OrWmMoPhUl5jm/FMNAnJvWQ=
 github.com/envoyproxy/go-control-plane v0.9.0/go.mod h1:YTl/9mNaCwkRvm6d1a2C3ymFceY/DCBVvsKhRF0iEA4=
 github.com/envoyproxy/go-control-plane v0.9.1-0.20191026205805-5f8ba28d4473/go.mod h1:YTl/9mNaCwkRvm6d1a2C3ymFceY/DCBVvsKhRF0iEA4=
 github.com/envoyproxy/go-control-plane v0.9.4/go.mod h1:6rpuAdCZL397s3pYoYcLgu1mIlRU8Am5FuJP05cCM98=
@@ -110,6 +126,17 @@
 github.com/felixge/httpsnoop v1.0.2 h1:+nS9g82KMXccJ/wp0zyRW9ZBHFETmMGtkk+2CTTrW4o=
 github.com/felixge/httpsnoop v1.0.2/go.mod h1:m8KPJKqk1gH5J9DgRY2ASl2lWCfGKXixSwevea8zH2U=
 github.com/ghodss/yaml v1.0.0/go.mod h1:4dBDuWmgqj2HViK6kFavaiC9ZROes6MMH2rRYeMEF04=
+github.com/gliderlabs/ssh v0.3.5 h1:OcaySEmAQJgyYcArR+gGGTHCyE7nvhEMTlYY+Dp8CpY=
+github.com/gliderlabs/ssh v0.3.5/go.mod h1:8XB4KraRrX39qHhT6yxPsHedjA08I/uBVwj4xC+/+z4=
+github.com/go-git/gcfg v1.5.0 h1:Q5ViNfGF8zFgyJWPqYwA7qGFoMTEiBmdlkcfRmpIMa4=
+github.com/go-git/gcfg v1.5.0/go.mod h1:5m20vg6GwYabIxaOonVkTdrILxQMpEShl1xiMF4ua+E=
+github.com/go-git/go-billy/v5 v5.3.1/go.mod h1:pmpqyWchKfYfrkb/UVH4otLvyi/5gJlGI4Hb3ZqZ3W0=
+github.com/go-git/go-billy/v5 v5.4.0 h1:Vaw7LaSTRJOUric7pe4vnzBSgyuf2KrLsu2Y4ZpQBDE=
+github.com/go-git/go-billy/v5 v5.4.0/go.mod h1:vjbugF6Fz7JIflbVpl1hJsGjSHNltrSw45YK/ukIvQg=
+github.com/go-git/go-git-fixtures/v4 v4.3.1 h1:y5z6dd3qi8Hl+stezc8p3JxDkoTRqMAlKnXHuzrfjTQ=
+github.com/go-git/go-git-fixtures/v4 v4.3.1/go.mod h1:8LHG1a3SRW71ettAD/jW13h8c6AqjVSeL11RAdgaqpo=
+github.com/go-git/go-git/v5 v5.5.2 h1:v8lgZa5k9ylUw+OR/roJHTxR4QItsNFI5nKtAXFuynw=
+github.com/go-git/go-git/v5 v5.5.2/go.mod h1:BE5hUJ5yaV2YMxhmaP4l6RBQ08kMxKSPD4BlxtH7OjI=
 github.com/go-gl/glfw v0.0.0-20190409004039-e6da0acd62b1/go.mod h1:vR7hzQXu2zJy9AVAgeJqvqgH9Q5CA+iKCZ2gyEVpxRU=
 github.com/go-gl/glfw/v3.3/glfw v0.0.0-20191125211704-12ad95a8df72/go.mod h1:tQ2UAYgL5IevRw8kRxooKSPJfGvJ9fJQFa0TUsXzTg8=
 github.com/go-gl/glfw/v3.3/glfw v0.0.0-20200222043503-6f7a984d4dc4/go.mod h1:tQ2UAYgL5IevRw8kRxooKSPJfGvJ9fJQFa0TUsXzTg8=
@@ -207,23 +234,47 @@
 github.com/hashicorp/golang-lru v0.5.1/go.mod h1:/m3WP610KZHVQ1SGc6re/UDhFvYD7pJ4Ao+sR/qLZy8=
 github.com/ianlancetaylor/demangle v0.0.0-20181102032728-5e5cf60278f6/go.mod h1:aSSvb/t6k1mPoxDqO4vJh6VOCGPwU4O0C2/Eqndh1Sc=
 github.com/ianlancetaylor/demangle v0.0.0-20200824232613-28f6c0f3b639/go.mod h1:aSSvb/t6k1mPoxDqO4vJh6VOCGPwU4O0C2/Eqndh1Sc=
+github.com/imdario/mergo v0.3.13 h1:lFzP57bqS/wsqKssCGmtLAb8A0wKjLGrve2q3PPVcBk=
+github.com/imdario/mergo v0.3.13/go.mod h1:4lJ1jqUDcsbIECGy0RUJAXNIhg+6ocWgb1ALK2O4oXg=
+github.com/jbenet/go-context v0.0.0-20150711004518-d14ea06fba99 h1:BQSFePA1RWJOlocH6Fxy8MmwDt+yVQYULKfN0RoTN8A=
+github.com/jbenet/go-context v0.0.0-20150711004518-d14ea06fba99/go.mod h1:1lJo3i6rXxKeerYnT8Nvf0QmHCRC1n8sfWVwXF2Frvo=
+github.com/jessevdk/go-flags v1.5.0/go.mod h1:Fw0T6WPc1dYxT4mKEZRfG5kJhaTDP9pj1c2EWnYs/m4=
 github.com/jstemmer/go-junit-report v0.0.0-20190106144839-af01ea7f8024/go.mod h1:6v2b51hI/fHJwM22ozAgKL4VKDeJcHhJFhtBdhmNjmU=
 github.com/jstemmer/go-junit-report v0.9.1/go.mod h1:Brl9GWCQeLvo8nXZwPNNblvFj/XSXhF0NWZEnDohbsk=
+github.com/kevinburke/ssh_config v1.2.0 h1:x584FjTGwHzMwvHx18PXxbBVzfnxogHaAReU4gf13a4=
+github.com/kevinburke/ssh_config v1.2.0/go.mod h1:CT57kijsi8u/K/BOFA39wgDQJ9CxiF4nAY/ojJ6r6mM=
 github.com/kisielk/gotool v1.0.0/go.mod h1:XhKaO+MFFWcvkIS/tQcRk01m1F5IRFswLeQ+oQHNcck=
 github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORNo=
+github.com/kr/pretty v0.2.1 h1:Fmg33tUaq4/8ym9TJN1x7sLJnHVwhP33CNkpYV/7rwI=
+github.com/kr/pretty v0.2.1/go.mod h1:ipq/a2n7PKx3OHsz4KJII5eveXtPO4qwEXGdVfWzfnI=
 github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
 github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
+github.com/kr/text v0.2.0 h1:5Nx0Ya0ZqY2ygV366QzturHI13Jq95ApcVaJBhpS+AY=
+github.com/kr/text v0.2.0/go.mod h1:eLer722TekiGuMkidMxC/pM04lWEeraHUUmBw8l2grE=
+github.com/matryer/is v1.2.0 h1:92UTHpy8CDwaJ08GqLDzhhuixiBUUD1p3AU6PHddz4A=
+github.com/matryer/is v1.2.0/go.mod h1:2fLPjFQM9rhQ15aVEtbuwhJinnOqrmgXPNdZsdwlWXA=
+github.com/niemeyer/pretty v0.0.0-20200227124842-a10e7caefd8e/go.mod h1:zD1mROLANZcx1PVRCS0qkT7pwLkGfwJo4zjcN/Tysno=
+github.com/pjbgf/sha1cd v0.2.3 h1:uKQP/7QOzNtKYH7UTohZLcjF5/55EnTw0jO/Ru4jZwI=
+github.com/pjbgf/sha1cd v0.2.3/go.mod h1:HOK9QrgzdHpbc2Kzip0Q1yi3M2MFGPADtR6HjG65m5M=
 github.com/pkg/diff v0.0.0-20210226163009-20ebb0f2a09e h1:aoZm08cpOy4WuID//EZDgcC4zIxODThtZNPirFr42+A=
+github.com/pkg/errors v0.9.1 h1:FEBLx1zS214owpjy7qsBeixbURkuhQAwrK5UwLGTwt4=
+github.com/pkg/errors v0.9.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/prometheus/client_model v0.0.0-20190812154241-14fe0d1b01d4/go.mod h1:xMI15A0UPsDsEKsMN9yxemIoYk6Tm2C1GtYGdfGttqA=
 github.com/rogpeppe/fastuuid v1.2.0/go.mod h1:jVj6XXZzXRy/MSR5jhDC/2q6DgLz+nrA6LYCDYWNEvQ=
 github.com/rogpeppe/go-internal v1.3.0/go.mod h1:M8bDsm7K2OlrFYOpmOWEs/qY81heoFRclV5y23lUDJ4=
 github.com/rogpeppe/go-internal v1.9.0 h1:73kH8U+JUqXU8lRuOHeVHaa/SZPifC7BkcraZVejAe8=
+github.com/sergi/go-diff v1.1.0 h1:we8PVUC3FE2uYfodKH/nBHMSetSfHDR6scGdBi+erh0=
+github.com/sergi/go-diff v1.1.0/go.mod h1:STckp+ISIX8hZLjrqAeVduY0gWCT9IjLuqbuNXdaHfM=
+github.com/sirupsen/logrus v1.7.0/go.mod h1:yWOB1SBYBC5VeMP7gHvWumXLIWorT60ONWic61uBYv0=
+github.com/skeema/knownhosts v1.1.0 h1:Wvr9V0MxhjRbl3f9nMnKnFfiWTJmtECJ9Njkea3ysW0=
+github.com/skeema/knownhosts v1.1.0/go.mod h1:sKFq3RD6/TKZkSWn8boUbDC7Qkgcv+8XXijpFO6roag=
 github.com/spaolacci/murmur3 v0.0.0-20180118202830-f09979ecbc72/go.mod h1:JwIasOWyU6f++ZhiEuf87xNszmSA2myDM2Kzu9HwQUA=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
 github.com/stretchr/objx v0.4.0/go.mod h1:YvHI0jy2hoMjB+UWwv71VJQ9isScKT/TqJzVSSt89Yw=
 github.com/stretchr/objx v0.5.0/go.mod h1:Yh+to48EsGEfYuaHDzXPcE3xhTkx73EhmCGUpEOglKo=
+github.com/stretchr/testify v1.2.2/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=
 github.com/stretchr/testify v1.4.0/go.mod h1:j7eGeouHqKxXV5pUuKE4zz7dFj8WfuZ+81PSLYec5m4=
 github.com/stretchr/testify v1.5.1/go.mod h1:5W2xD1RspED5o8YsWQXVCued0rvSQ+mT+I5cxcmMvtA=
 github.com/stretchr/testify v1.6.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
@@ -232,11 +283,14 @@
 github.com/stretchr/testify v1.8.0/go.mod h1:yNjHg4UonilssWZ8iaSj1OCr/vHnekPRkoO+kdMU+MU=
 github.com/stretchr/testify v1.8.1 h1:w7B6lhMri9wdJUVmEZPGGhZzrYTPvgJArz7wNPgYKsk=
 github.com/stretchr/testify v1.8.1/go.mod h1:w2LPCIKwWwSfY2zedu0+kehJoqGctiVI29o6fzry7u4=
+github.com/xanzy/ssh-agent v0.3.3 h1:+/15pJfg/RsTxqYcX6fHqOXZwwMP+2VyYWJeWM2qQFM=
+github.com/xanzy/ssh-agent v0.3.3/go.mod h1:6dzNDKs0J9rVPHPhaGCukekBHKqfl+L3KghI1Bc68Uw=
 github.com/yuin/goldmark v1.1.25/go.mod h1:3hX8gzYuyVAZsxl0MRgGTJEmQBFcNTphYh9decYSb74=
 github.com/yuin/goldmark v1.1.27/go.mod h1:3hX8gzYuyVAZsxl0MRgGTJEmQBFcNTphYh9decYSb74=
 github.com/yuin/goldmark v1.1.32/go.mod h1:3hX8gzYuyVAZsxl0MRgGTJEmQBFcNTphYh9decYSb74=
 github.com/yuin/goldmark v1.2.1/go.mod h1:3hX8gzYuyVAZsxl0MRgGTJEmQBFcNTphYh9decYSb74=
 github.com/yuin/goldmark v1.3.5/go.mod h1:mwnBkeHKe2W/ZEtQ+71ViKU8L12m81fl3OWwC1Zlc8k=
+github.com/yuin/goldmark v1.4.13/go.mod h1:6yULJ656Px+3vBD8DxQVa3kxgyrAnzto9xy5taEt/CY=
 go.opencensus.io v0.21.0/go.mod h1:mSImk1erAIZhrmZN+AvHh14ztQfjbGwt4TtuofqLduU=
 go.opencensus.io v0.22.0/go.mod h1:+kGneAE2xo2IficOXnaByMWTGM9T73dGwxeWcUqIpI8=
 go.opencensus.io v0.22.2/go.mod h1:yxeiOL68Rb0Xd1ddK5vPZ/oVn4vY4Ynel7k9FzqtOIw=
@@ -282,6 +336,12 @@
 golang.org/x/crypto v0.0.0-20190605123033-f99c8df09eb5/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
 golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
 golang.org/x/crypto v0.0.0-20200622213623-75b288015ac9/go.mod h1:LzIPMQfyMNhhGPhUkYOs5KpL4U8rLKemX1yGLhDgUto=
+golang.org/x/crypto v0.0.0-20210921155107-089bfa567519/go.mod h1:GvvjBRRGRdwPK5ydBHafDWAxML/pGHZbMvKqRZ5+Abc=
+golang.org/x/crypto v0.0.0-20220525230936-793ad666bf5e/go.mod h1:IxCIyHEi3zRg3s0A5j5BB6A9Jmi73HwBIUl50j+osU4=
+golang.org/x/crypto v0.0.0-20220622213112-05595931fe9d/go.mod h1:IxCIyHEi3zRg3s0A5j5BB6A9Jmi73HwBIUl50j+osU4=
+golang.org/x/crypto v0.0.0-20220826181053-bd7e27e6170d/go.mod h1:IxCIyHEi3zRg3s0A5j5BB6A9Jmi73HwBIUl50j+osU4=
+golang.org/x/crypto v0.3.0 h1:a06MkbcxBrEFc0w0QIZWXrH/9cCX6KJyWbBOIwAn+7A=
+golang.org/x/crypto v0.3.0/go.mod h1:hebNnKkNXi2UzZN1eVRvBB7co0a+JxK6XbPiWVs/3J4=
 golang.org/x/exp v0.0.0-20190121172915-509febef88a4/go.mod h1:CJ0aWSM057203Lf6IL+f9T1iT9GByDxfZKAQTCR3kQA=
 golang.org/x/exp v0.0.0-20190306152737-a1d7652674e8/go.mod h1:CJ0aWSM057203Lf6IL+f9T1iT9GByDxfZKAQTCR3kQA=
 golang.org/x/exp v0.0.0-20190510132918-efd6b22b2522/go.mod h1:ZjyILWgesfNpC6sMxTJOJm9Kp84zZh5NQWvqDGG3Qr8=
@@ -319,6 +379,7 @@
 golang.org/x/mod v0.4.0/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
 golang.org/x/mod v0.4.1/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
 golang.org/x/mod v0.4.2/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
+golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4/go.mod h1:jJ57K6gSWd91VN4djpZkiMVwK6gcyfeH4XE8wZrZaV4=
 golang.org/x/mod v0.7.0 h1:LapD9S96VoQRhi/GrNTqeBJFrUjs5UHCAtTlgwA5oZA=
 golang.org/x/mod v0.7.0/go.mod h1:iBbtSCu2XBx23ZKBPSOrRkjjQPZFPuis4dIYUhu/chs=
 golang.org/x/net v0.0.0-20180724234803-3673e40ba225/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
@@ -358,7 +419,11 @@
 golang.org/x/net v0.0.0-20210405180319-a5a99cb37ef4/go.mod h1:p54w0d4576C0XHj96bSt6lcn1PtDYWL6XObtHCRCNQM=
 golang.org/x/net v0.0.0-20210503060351-7fd8e65b6420/go.mod h1:9nx3DQGgdP8bBQD5qxJ1jj9UTztislL4KSBs9R2vV5Y=
 golang.org/x/net v0.0.0-20210716203947-853a461950ff/go.mod h1:9nx3DQGgdP8bBQD5qxJ1jj9UTztislL4KSBs9R2vV5Y=
+golang.org/x/net v0.0.0-20211112202133-69e39bad7dc2/go.mod h1:9nx3DQGgdP8bBQD5qxJ1jj9UTztislL4KSBs9R2vV5Y=
 golang.org/x/net v0.0.0-20220127200216-cd36cc0744dd/go.mod h1:CfG3xpIq0wQ8r1q4Su4UZFWDARRcnwPjda9FqA0JpMk=
+golang.org/x/net v0.0.0-20220722155237-a158d28d115b/go.mod h1:XRhObCWvk6IyKnWLug+ECip1KBveYUHfp+8e9klMJ9c=
+golang.org/x/net v0.0.0-20220826154423-83b083e8dc8b/go.mod h1:YDH+HFinaLZZlnHAfSS6ZXJJ9M9t4Dl22yv3iI2vPwk=
+golang.org/x/net v0.2.0/go.mod h1:KqCZLdyyvdV855qA2rE3GC2aiw5xGR5TEjj8smXukLY=
 golang.org/x/net v0.5.0 h1:GyT4nK/YDHSqa1c4753ouYCDajOYKTja9Xb/OHtgvSw=
 golang.org/x/net v0.5.0/go.mod h1:DivGGAXEgPSlEBzxGzZI+ZLohi+xUj054jfeKui00ws=
 golang.org/x/oauth2 v0.0.0-20180821212333-d2e6202438be/go.mod h1:N/0e6XlmueqKjAGxoOufVs8QHGRruUQn6yWY3a++T0U=
@@ -390,6 +455,7 @@
 golang.org/x/sync v0.0.0-20201020160332-67f06af15bc9/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.0.0-20201207232520-09787c993a3a/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.0.0-20210220032951-036812b2e83c/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.0.0-20220722155255-886fb9371eb4/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.1.0 h1:wsuoTGHzEhffawBOhz5CYhcrV4IdKZbEyZjBMuTp12o=
 golang.org/x/sync v0.1.0/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sys v0.0.0-20180830151530-49385e6e1522/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
@@ -402,6 +468,7 @@
 golang.org/x/sys v0.0.0-20190624142023-c5567b49c5d0/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20190726091711-fc99dfbffb4e/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20191001151750-bb3f8db39f24/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20191026070338-33540a1f6037/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20191204072324-ce4227a45e2e/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20191228213918-04cbcbbfeed8/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20200113162924-86b910548bc1/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
@@ -423,6 +490,7 @@
 golang.org/x/sys v0.0.0-20201201145000-ef89a241ccb3/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210104204734-6f8348627aad/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210119212857-b64e53b001e4/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20210124154548-22da62e12c0c/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210220050731-9a76102bfb43/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210305230114-8fe3ee5dd75b/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210315160823-c6e025ad8005/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
@@ -439,16 +507,27 @@
 golang.org/x/sys v0.0.0-20210806184541-e5e7981a1069/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20210823070655-63515b42dcdf/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20210908233432-aa78b53d3365/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20211007075335-d3039528d8ac/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20211124211545-fe61309f8881/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20211210111614-af8b64212486/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20211216021012-1d35b9e2eb4e/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20220114195835-da31bd327af9/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20220204135822-1c1b9b1eba6a/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20220207234003-57398862261d/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20220715151400-c0bba94af5f8/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20220722155257-8c9f86f7a55f/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20220728004956-3c1f35247d10/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20220825204002-c680a09ffe64/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.2.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.3.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.4.0 h1:Zr2JFtRQNX3BCZ8YtxRE9hNJYC8J6I1MVbMg6owUp18=
 golang.org/x/sys v0.4.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
 golang.org/x/term v0.0.0-20210927222741-03fcf44c2211/go.mod h1:jbD1KX2456YbFQfuXm/mYQcufACuNUgVhRMnK/tPxf8=
+golang.org/x/term v0.0.0-20220722155259-a9ba230a4035/go.mod h1:jbD1KX2456YbFQfuXm/mYQcufACuNUgVhRMnK/tPxf8=
+golang.org/x/term v0.2.0/go.mod h1:TVmDHMZPmdnySmBfhjOoOdhjzdE1h4u1VwSiw2l1Nuc=
+golang.org/x/term v0.4.0 h1:O7UWfv5+A2qiuulQk30kVinPoMtoIPeVaKLEgLpVkvg=
 golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/text v0.3.1-0.20180807135948-17ff2d5776d2/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
@@ -458,6 +537,7 @@
 golang.org/x/text v0.3.5/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
 golang.org/x/text v0.3.6/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
 golang.org/x/text v0.3.7/go.mod h1:u+2+/6zg+i71rQMx5EYifcz6MCKuco9NR6JIITiCfzQ=
+golang.org/x/text v0.4.0/go.mod h1:mrYo+phRRbMaCq/xk9113O4dZlRixOauAjOtrjsXDZ8=
 golang.org/x/text v0.6.0 h1:3XmdazWV+ubf7QgHSTWeykHOci5oeekaGJBLkrkaw4k=
 golang.org/x/text v0.6.0/go.mod h1:mrYo+phRRbMaCq/xk9113O4dZlRixOauAjOtrjsXDZ8=
 golang.org/x/time v0.0.0-20181108054448-85acf8d2951c/go.mod h1:tRJNPiyCQ0inRvYxbN9jk5I+vvW/OXSQhTDSoE431IQ=
@@ -515,6 +595,7 @@
 golang.org/x/tools v0.1.3/go.mod h1:o0xws9oXOQQZyjljx8fwUC0k7L1pTE6eaCbjGeHmOkk=
 golang.org/x/tools v0.1.4/go.mod h1:o0xws9oXOQQZyjljx8fwUC0k7L1pTE6eaCbjGeHmOkk=
 golang.org/x/tools v0.1.5/go.mod h1:o0xws9oXOQQZyjljx8fwUC0k7L1pTE6eaCbjGeHmOkk=
+golang.org/x/tools v0.1.12/go.mod h1:hNGJHUnrk76NpqgfD5Aqm5Crs+Hm0VOH/i9J2+nxYbc=
 golang.org/x/tools v0.5.1-0.20230117180257-8aba49bb5ea2 h1:v0FhRDmSCNH/0EurAT6T8KRY4aNuUhz6/WwBMxG+gvQ=
 golang.org/x/tools v0.5.1-0.20230117180257-8aba49bb5ea2/go.mod h1:N+Kgy78s5I24c24dU8OfWNEotWjutIs8SnJvn5IDq+k=
 golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
@@ -681,10 +762,17 @@
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/check.v1 v1.0.0-20190902080502-41f04d3bba15/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/check.v1 v1.0.0-20200227125254-8fa46927fb4f/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c h1:Hei/4ADfdWqJk1ZMxUNpqntNwaWcugrBjAiHlqqRiVk=
+gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c/go.mod h1:JHkPIbrfpd72SG/EVd6muEfDQjcINNoR0C8j2r3qZ4Q=
 gopkg.in/errgo.v2 v2.1.0/go.mod h1:hNsd1EY+bozCKY1Ytp96fpM3vjJbqLJn88ws8XvfDNI=
+gopkg.in/warnings.v0 v0.1.2 h1:wFXVbFY8DY5/xOe1ECiWdKCzZlxgshcYVNkBHstARME=
+gopkg.in/warnings.v0 v0.1.2/go.mod h1:jksf8JmL6Qr/oQM2OXTHunEvvTAsrWBLb6OOjuVWRNI=
 gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v2 v2.2.3/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
+gopkg.in/yaml.v2 v2.2.4/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
+gopkg.in/yaml.v3 v3.0.0/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 honnef.co/go/tools v0.0.0-20190102054323-c2f93a96b099/go.mod h1:rf3lG4BRIbNafJWhAfAdb/ePZxsR/4RtNHQocxwk9r4=
diff --git a/internal/stdlib/gorepo.go b/internal/stdlib/gorepo.go
new file mode 100644
index 0000000..7709384
--- /dev/null
+++ b/internal/stdlib/gorepo.go
@@ -0,0 +1,179 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package stdlib
+
+import (
+	"fmt"
+	"path/filepath"
+
+	"github.com/go-git/go-billy/v5/osfs"
+	"github.com/go-git/go-git/v5"
+	"github.com/go-git/go-git/v5/config"
+	"github.com/go-git/go-git/v5/plumbing"
+	"github.com/go-git/go-git/v5/plumbing/object"
+	"github.com/go-git/go-git/v5/storage/memory"
+	"golang.org/x/pkgsite-metrics/internal/derrors"
+	"golang.org/x/pkgsite-metrics/internal/testing/testhelper"
+	"golang.org/x/pkgsite-metrics/internal/version"
+)
+
+// A goRepo represents a git repo holding the Go standard library.
+type goRepo interface {
+	// Return the repo at the given version.
+	repoAtVersion(version string) (*git.Repository, plumbing.ReferenceName, error)
+
+	// Return all the refs of the repo.
+	refs() ([]*plumbing.Reference, error)
+}
+
+type remoteGoRepo struct{}
+
+// repoAtVersion returns a repo object for the Go repo at version by cloning the
+// Go repo.
+func (remoteGoRepo) repoAtVersion(v string) (_ *git.Repository, ref plumbing.ReferenceName, err error) {
+	defer derrors.Wrap(&err, "remoteGoRepo.repoAtVersion(%q)", v)
+
+	ref, err = refNameForVersion(v)
+	if err != nil {
+		return nil, "", err
+	}
+	repo, err := git.Clone(memory.NewStorage(), nil, &git.CloneOptions{
+		URL:           GoRepoURL,
+		ReferenceName: ref,
+		SingleBranch:  true,
+		Depth:         1,
+		Tags:          git.NoTags,
+	})
+	if err != nil {
+		return nil, "", err
+	}
+	return repo, ref, nil
+}
+
+func (remoteGoRepo) refs() (_ []*plumbing.Reference, err error) {
+	defer derrors.Wrap(&err, "remoteGoRepo.refs")
+
+	re := git.NewRemote(memory.NewStorage(), &config.RemoteConfig{
+		URLs: []string{GoRepoURL},
+	})
+	return re.List(&git.ListOptions{})
+}
+
+type localGoRepo struct {
+	path string
+	repo *git.Repository
+}
+
+func newLocalGoRepo(path string) (*localGoRepo, error) {
+	repo, err := git.PlainOpen(path)
+	if err != nil {
+		return nil, err
+	}
+	return &localGoRepo{
+		path: path,
+		repo: repo,
+	}, nil
+}
+
+func (g *localGoRepo) repoAtVersion(v string) (_ *git.Repository, ref plumbing.ReferenceName, err error) {
+	defer derrors.Wrap(&err, "localGoRepo(%s).repoAtVersion(%q)", g.path, v)
+	ref, err = refNameForVersion(v)
+	if err != nil {
+		return nil, "", err
+	}
+	return g.repo, ref, nil
+}
+
+func (g *localGoRepo) refs() (rs []*plumbing.Reference, err error) {
+	defer derrors.Wrap(&err, "localGoRepo(%s).refs", g.path)
+
+	iter, err := g.repo.References()
+	if err != nil {
+		return nil, err
+	}
+	defer iter.Close()
+	err = iter.ForEach(func(r *plumbing.Reference) error {
+		rs = append(rs, r)
+		return nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	return rs, nil
+}
+
+type testGoRepo struct {
+}
+
+// repoAtVersion gets a Go repo for testing.
+func (t *testGoRepo) repoAtVersion(v string) (_ *git.Repository, ref plumbing.ReferenceName, err error) {
+	defer derrors.Wrap(&err, "testGoRepo.repoAtVersion(%q)", v)
+	if v == TestMasterVersion {
+		v = version.Master
+	}
+	if v == TestDevFuzzVersion {
+		v = DevFuzz
+	}
+	fs := osfs.New(filepath.Join(testhelper.TestDataPath("testdata"), v))
+	repo, err := git.Init(memory.NewStorage(), fs)
+	if err != nil {
+		return nil, "", fmt.Errorf("git.Init: %v", err)
+	}
+	wt, err := repo.Worktree()
+	if err != nil {
+		return nil, "", fmt.Errorf("repo.Worktree: %v", err)
+	}
+	// Add all files in the directory.
+	if _, err := wt.Add(""); err != nil {
+		return nil, "", fmt.Errorf("wt.Add(): %v", err)
+	}
+	_, err = wt.Commit("", &git.CommitOptions{All: true, Author: &object.Signature{
+		Name:  "Joe Random",
+		Email: "joe@example.com",
+		When:  TestCommitTime,
+	}})
+	if err != nil {
+		return nil, "", fmt.Errorf("wt.Commit: %v", err)
+	}
+	return repo, plumbing.HEAD, nil
+}
+
+// References used for Versions during testing.
+var testRefs = []plumbing.ReferenceName{
+	// stdlib versions
+	"refs/tags/go1.2.1",
+	"refs/tags/go1.3.2",
+	"refs/tags/go1.4.2",
+	"refs/tags/go1.4.3",
+	"refs/tags/go1.6",
+	"refs/tags/go1.6.3",
+	"refs/tags/go1.6beta1",
+	"refs/tags/go1.8",
+	"refs/tags/go1.8rc2",
+	"refs/tags/go1.9rc1",
+	"refs/tags/go1.11",
+	"refs/tags/go1.12",
+	"refs/tags/go1.12.1",
+	"refs/tags/go1.12.5",
+	"refs/tags/go1.12.9",
+	"refs/tags/go1.13",
+	"refs/tags/go1.13beta1",
+	"refs/tags/go1.14.6",
+	"refs/heads/dev.fuzz",
+	"refs/heads/master",
+	// other tags
+	"refs/changes/56/93156/13",
+	"refs/tags/release.r59",
+	"refs/tags/weekly.2011-04-13",
+}
+
+func (t *testGoRepo) refs() ([]*plumbing.Reference, error) {
+	var rs []*plumbing.Reference
+	for _, r := range testRefs {
+		// Only the name is ever used, so the referent can be empty.
+		rs = append(rs, plumbing.NewSymbolicReference(r, ""))
+	}
+	return rs, nil
+}
diff --git a/internal/stdlib/stdlib.go b/internal/stdlib/stdlib.go
new file mode 100644
index 0000000..82b4310
--- /dev/null
+++ b/internal/stdlib/stdlib.go
@@ -0,0 +1,556 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package stdlib supports special handling of the Go standard library.
+// Regardless of the how the standard library has been split into modules for
+// development and testing, the discovery site treats it as a single module
+// named "std".
+package stdlib
+
+import (
+	"archive/zip"
+	"bytes"
+	"fmt"
+	"io"
+	"io/fs"
+	"os"
+	"path"
+	"regexp"
+	"strings"
+	"sync"
+	"time"
+
+	"golang.org/x/mod/semver"
+	"golang.org/x/pkgsite-metrics/internal/derrors"
+	"golang.org/x/pkgsite-metrics/internal/version"
+
+	"github.com/go-git/go-git/v5"
+	"github.com/go-git/go-git/v5/plumbing"
+	"github.com/go-git/go-git/v5/plumbing/filemode"
+	"github.com/go-git/go-git/v5/plumbing/object"
+)
+
+const (
+	// ModulePath is the name of the module for the standard library.
+	ModulePath = "std"
+
+	// DevFuzz is the branch name for fuzzing in beta.
+	DevFuzz = "dev.fuzz"
+
+	// DevBoringCrypto is the branch name for dev.boringcrypto.
+	DevBoringCrypto = "dev.boringcrypto"
+)
+
+var (
+	// Regexp for matching go tags. The groups are:
+	// 1  the major.minor version
+	// 2  the patch version, or empty if none
+	// 3  the entire prerelease, if present
+	// 4  the prerelease type ("beta" or "rc")
+	// 5  the prerelease number
+	tagRegexp = regexp.MustCompile(`^go(\d+\.\d+)(\.\d+|)((beta|rc)(\d+))?$`)
+)
+
+// SupportedBranches are the branches of the stdlib repo supported by pkgsite.
+var SupportedBranches = map[string]bool{
+	version.Master:  true,
+	DevBoringCrypto: true,
+	DevFuzz:         true,
+}
+
+// VersionForTag returns the semantic version for the Go tag, or "" if
+// tag doesn't correspond to a Go release or beta tag. In special cases,
+// when the tag specified is either `latest` or `master` it will return the tag.
+// Examples:
+//
+//	"go1" => "v1.0.0"
+//	"go1.2" => "v1.2.0"
+//	"go1.13beta1" => "v1.13.0-beta.1"
+//	"go1.9rc2" => "v1.9.0-rc.2"
+//	"latest" => "latest"
+//	"master" => "master"
+func VersionForTag(tag string) string {
+	// Special cases for go1.
+	if tag == "go1" {
+		return "v1.0.0"
+	}
+	if tag == "go1.0" {
+		return ""
+	}
+	// Special case for latest and master.
+	if tag == version.Latest || SupportedBranches[tag] {
+		return tag
+	}
+	m := tagRegexp.FindStringSubmatch(tag)
+	if m == nil {
+		return ""
+	}
+	version := "v" + m[1]
+	if m[2] != "" {
+		version += m[2]
+	} else {
+		version += ".0"
+	}
+	if m[3] != "" {
+		version += "-" + m[4] + "." + m[5]
+	}
+	return version
+}
+
+// TagForVersion returns the Go standard library repository tag corresponding
+// to semver. The Go tags differ from standard semantic versions in a few ways,
+// such as beginning with "go" instead of "v".
+func TagForVersion(v string) (_ string, err error) {
+	defer derrors.Wrap(&err, "TagForVersion(%q)", v)
+
+	// Special case: master => master or dev.fuzz => dev.fuzz
+	if SupportedBranches[v] {
+		return v, nil
+	}
+	if strings.HasPrefix(v, "v0.0.0") {
+		return version.Master, nil
+	}
+	// Special case: v1.0.0 => go1.
+	if v == "v1.0.0" {
+		return "go1", nil
+	}
+	if !semver.IsValid(v) {
+		return "", fmt.Errorf("%w: requested version is not a valid semantic version: %q ", derrors.InvalidArgument, v)
+	}
+	goVersion := semver.Canonical(v)
+	prerelease := semver.Prerelease(goVersion)
+	versionWithoutPrerelease := strings.TrimSuffix(goVersion, prerelease)
+	patch := strings.TrimPrefix(versionWithoutPrerelease, semver.MajorMinor(goVersion)+".")
+	if patch == "0" {
+		versionWithoutPrerelease = strings.TrimSuffix(versionWithoutPrerelease, ".0")
+	}
+	goVersion = fmt.Sprintf("go%s", strings.TrimPrefix(versionWithoutPrerelease, "v"))
+	if prerelease != "" {
+		// Go prereleases look like  "beta1" instead of "beta.1".
+		// "beta1" is bad for sorting (since beta10 comes before beta9), so
+		// require the dot form.
+		i := finalDigitsIndex(prerelease)
+		if i >= 1 {
+			if prerelease[i-1] != '.' {
+				return "", fmt.Errorf("%w: final digits in a prerelease must follow a period", derrors.InvalidArgument)
+			}
+			// Remove the dot.
+			prerelease = prerelease[:i-1] + prerelease[i:]
+		}
+		goVersion += strings.TrimPrefix(prerelease, "-")
+	}
+	return goVersion, nil
+}
+
+// MajorVersionForVersion returns the Go major version for version.
+// E.g. "v1.13.3" => "go1".
+func MajorVersionForVersion(version string) (_ string, err error) {
+	defer derrors.Wrap(&err, "MajorVersionForVersion(%q)", version)
+
+	tag, err := TagForVersion(version)
+	if err != nil {
+		return "", err
+	}
+	if tag == "go1" {
+		return tag, nil
+	}
+	i := strings.IndexRune(tag, '.')
+	if i < 0 {
+		return "", fmt.Errorf("no '.' in go tag %q", tag)
+	}
+	return tag[:i], nil
+}
+
+// finalDigitsIndex returns the index of the first digit in the sequence of digits ending s.
+// If s doesn't end in digits, it returns -1.
+func finalDigitsIndex(s string) int {
+	// Assume ASCII (since the semver package does anyway).
+	var i int
+	for i = len(s) - 1; i >= 0; i-- {
+		if s[i] < '0' || s[i] > '9' {
+			break
+		}
+	}
+	if i == len(s)-1 {
+		return -1
+	}
+	return i + 1
+}
+
+const (
+	GoRepoURL       = "https://go.googlesource.com/go"
+	GoSourceRepoURL = "https://cs.opensource.google/go/go"
+
+	GitHubRepo = "github.com/golang/go"
+)
+
+// TestCommitTime is the time used for all commits when UseTestData is true.
+var (
+	TestCommitTime     = time.Date(2019, 9, 4, 1, 2, 3, 0, time.UTC)
+	TestMasterVersion  = "v0.0.0-20190904010203-89fb59e2e920"
+	TestDevFuzzVersion = "v0.0.0-20190904010203-12de34vf56uz"
+)
+
+var (
+	goRepoMu  sync.Mutex
+	theGoRepo goRepo = &remoteGoRepo{}
+)
+
+func getGoRepo() goRepo {
+	goRepoMu.Lock()
+	defer goRepoMu.Unlock()
+	return theGoRepo
+}
+
+func swapGoRepo(gr goRepo) goRepo {
+	goRepoMu.Lock()
+	defer goRepoMu.Unlock()
+	old := theGoRepo
+	theGoRepo = gr
+	return old
+}
+
+// WithTestData arranges for this package to use a testing version of the Go repo.
+// The returned function restores the previous state. Use with defer:
+//
+//	defer WithTestData()()
+func WithTestData() func() {
+	return withGoRepo(&testGoRepo{})
+}
+
+func withGoRepo(gr goRepo) func() {
+	old := swapGoRepo(gr)
+	return func() {
+		swapGoRepo(old)
+	}
+}
+
+// SetGoRepoPath tells this package to obtain the Go repo from the
+// local filesystem at path, instead of cloning it.
+func SetGoRepoPath(path string) error {
+	gr, err := newLocalGoRepo(path)
+	if err != nil {
+		return err
+	}
+	swapGoRepo(gr)
+	return nil
+}
+
+func refNameForVersion(v string) (plumbing.ReferenceName, error) {
+	if v == version.Master {
+		return plumbing.HEAD, nil
+	}
+	if SupportedBranches[v] {
+		return plumbing.NewBranchReferenceName(v), nil
+	}
+	tag, err := TagForVersion(v)
+	if err != nil {
+		return "", err
+	}
+	return plumbing.NewTagReferenceName(tag), nil
+}
+
+// Versions returns all the versions of Go that are relevant to the discovery
+// site. These are all release versions (tags of the forms "goN.N" and
+// "goN.N.N", where N is a number) and beta or rc versions (tags of the forms
+// "goN.NbetaN" and "goN.N.NbetaN", and similarly for "rc" replacing "beta").
+func Versions() (_ []string, err error) {
+	defer derrors.Wrap(&err, "stdlib.Versions()")
+
+	refs, err := getGoRepo().refs()
+	if err != nil {
+		return nil, err
+	}
+	var versions []string
+	for _, r := range refs {
+		v := VersionForTag(r.Name().Short())
+		if v != "" {
+			versions = append(versions, v)
+		}
+	}
+	return versions, nil
+}
+
+// ResolveSupportedBranches returns the current hashes for each ref in
+// SupportedBranches.
+func ResolveSupportedBranches() (_ map[string]string, err error) {
+	defer derrors.Wrap(&err, "ResolveSupportedBranches")
+
+	refs, err := getGoRepo().refs()
+	if err != nil {
+		return nil, err
+	}
+	m := map[string]string{}
+	for _, r := range refs {
+		name := r.Name().Short()
+		if SupportedBranches[name] {
+			m[name] = r.Hash().String()
+		}
+	}
+	return m, nil
+}
+
+// Directory returns the directory of the standard library relative to the repo root.
+func Directory(v string) string {
+	if semver.Compare(v, "v1.4.0-beta.1") >= 0 ||
+		SupportedBranches[v] || strings.HasPrefix(v, "v0.0.0") {
+		return "src"
+	}
+	// For versions older than v1.4.0-beta.1, the stdlib is in src/pkg.
+	return "src/pkg"
+}
+
+// EstimatedZipSize is the approximate size of
+// Zip("v1.15.2").
+const EstimatedZipSize = 16 * 1024 * 1024
+
+// ZipInfo returns the proxy .info information for the module std.
+func ZipInfo(requestedVersion string) (resolvedVersion string, err error) {
+	defer derrors.Wrap(&err, "stdlib.ZipInfo(%q)", requestedVersion)
+
+	resolvedVersion, err = semanticVersion(requestedVersion)
+	if err != nil {
+		return "", err
+	}
+	return resolvedVersion, nil
+}
+
+func zipInternal(requestedVersion string) (_ *zip.Reader, resolvedVersion string, commitTime time.Time, prefix string, err error) {
+	if requestedVersion == version.Latest {
+		requestedVersion, err = semanticVersion(requestedVersion)
+		if err != nil {
+			return nil, "", time.Time{}, "", err
+		}
+	}
+	repo, refName, err := getGoRepo().repoAtVersion(requestedVersion)
+	if err != nil {
+		return nil, "", time.Time{}, "", err
+	}
+	var buf bytes.Buffer
+	z := zip.NewWriter(&buf)
+
+	ref, err := repo.Reference(refName, true)
+	if err != nil {
+		return nil, "", time.Time{}, "", err
+	}
+	commit, err := repo.CommitObject(ref.Hash())
+	if err != nil {
+		return nil, "", time.Time{}, "", err
+	}
+	resolvedVersion = requestedVersion
+	if SupportedBranches[requestedVersion] {
+		resolvedVersion = newPseudoVersion("v0.0.0", commit.Committer.When, commit.Hash)
+	}
+	root, err := repo.TreeObject(commit.TreeHash)
+	if err != nil {
+		return nil, "", time.Time{}, "", err
+	}
+	prefixPath := ModulePath + "@" + requestedVersion
+	// Add top-level files.
+	if err := addFiles(z, repo, root, prefixPath, false); err != nil {
+		return nil, "", time.Time{}, "", err
+	}
+	// Add files from the stdlib directory.
+	libdir := root
+	for _, d := range strings.Split(Directory(resolvedVersion), "/") {
+		libdir, err = subTree(repo, libdir, d)
+		if err != nil {
+			return nil, "", time.Time{}, "", err
+		}
+	}
+	if err := addFiles(z, repo, libdir, prefixPath, true); err != nil {
+		return nil, "", time.Time{}, "", err
+	}
+	if err := z.Close(); err != nil {
+		return nil, "", time.Time{}, "", err
+	}
+	br := bytes.NewReader(buf.Bytes())
+	zr, err := zip.NewReader(br, int64(br.Len()))
+	if err != nil {
+		return nil, "", time.Time{}, "", err
+	}
+	return zr, resolvedVersion, commit.Committer.When, prefixPath, nil
+}
+
+// ContentDir creates an fs.FS representing the entire Go standard library at the
+// given version (which must have been resolved with ZipInfo) and returns a
+// reader to it. It also returns the time of the commit for that version.
+//
+// Normally, ContentDir returns the resolved version it was passed. If the
+// resolved version is a supported branch like "master", ContentDir returns a
+// semantic version for the branch.
+//
+// ContentDir reads the standard library at the Go repository tag corresponding
+// to to the given semantic version.
+//
+// ContentDir ignores go.mod files in the standard library, treating it as if it
+// were a single module named "std" at the given version.
+func ContentDir(requestedVersion string) (_ fs.FS, resolvedVersion string, commitTime time.Time, err error) {
+	defer derrors.Wrap(&err, "stdlib.ContentDir(%q)", requestedVersion)
+
+	zr, resolvedVersion, commitTime, prefix, err := zipInternal(requestedVersion)
+	if err != nil {
+		return nil, "", time.Time{}, err
+	}
+	cdir, err := fs.Sub(zr, prefix)
+	if err != nil {
+		return nil, "", time.Time{}, err
+	}
+	return cdir, resolvedVersion, commitTime, nil
+}
+
+const pseudoHashLen = 12
+
+func newPseudoVersion(version string, commitTime time.Time, hash plumbing.Hash) string {
+	return fmt.Sprintf("%s-%s-%s", version, commitTime.Format("20060102150405"), hash.String()[:pseudoHashLen])
+}
+
+// VersionMatchesHash reports whether v is a pseudo-version whose hash
+// part matches the prefix of the given hash.
+func VersionMatchesHash(v, hash string) bool {
+	if !version.IsPseudo(v) {
+		return false
+	}
+	return v[len(v)-pseudoHashLen:] == hash[:pseudoHashLen]
+}
+
+// semanticVersion returns the semantic version corresponding to the
+// requestedVersion. If the requested version is version.Master, then semanticVersion
+// returns it as is. The branch name is resolved to a proper pseudo-version in
+// Zip.
+func semanticVersion(requestedVersion string) (_ string, err error) {
+	defer derrors.Wrap(&err, "semanticVersion(%q)", requestedVersion)
+
+	if SupportedBranches[requestedVersion] {
+		return requestedVersion, nil
+	}
+
+	knownVersions, err := Versions()
+	if err != nil {
+		return "", err
+	}
+
+	switch requestedVersion {
+	case version.Latest:
+		var latestVersion string
+		for _, v := range knownVersions {
+			if !strings.HasPrefix(v, "v") {
+				continue
+			}
+			versionType, err := version.ParseType(v)
+			if err != nil {
+				return "", err
+			}
+			if versionType != version.TypeRelease {
+				// We expect there to always be at least 1 release version.
+				continue
+			}
+			if semver.Compare(v, latestVersion) > 0 {
+				latestVersion = v
+			}
+		}
+		return latestVersion, nil
+	default:
+		for _, v := range knownVersions {
+			if v == requestedVersion {
+				return requestedVersion, nil
+			}
+		}
+	}
+
+	return "", fmt.Errorf("%w: requested version unknown: %q", derrors.InvalidArgument, requestedVersion)
+}
+
+// addFiles adds the files in t to z, using dirpath as the path prefix.
+// If recursive is true, it also adds the files in all subdirectories.
+func addFiles(z *zip.Writer, r *git.Repository, t *object.Tree, dirpath string, recursive bool) (err error) {
+	defer derrors.Wrap(&err, "addFiles(zip, repository, tree, %q, %t)", dirpath, recursive)
+
+	for _, e := range t.Entries {
+		if strings.HasPrefix(e.Name, ".") || strings.HasPrefix(e.Name, "_") {
+			continue
+		}
+		if e.Name == "go.mod" {
+			// Ignore; we don't need it.
+			continue
+		}
+		if strings.HasPrefix(e.Name, "README") && !strings.Contains(dirpath, "/") {
+			// For versions newer than v1.4.0-beta.1, the stdlib is in src/pkg.
+			// This means that our construction of the zip files will return
+			// two READMEs at the root:
+			// https://golang.org/README.md and
+			// https://golang.org/src/README.vendor
+			//
+			// We do not want to display the README.md
+			// or any README.vendor.
+			// However, we do want to store the README in
+			// other directories.
+			continue
+		}
+		switch e.Mode {
+		case filemode.Regular, filemode.Executable:
+			blob, err := r.BlobObject(e.Hash)
+			if err != nil {
+				return err
+			}
+			src, err := blob.Reader()
+			if err != nil {
+				return err
+			}
+			if err := writeZipFile(z, path.Join(dirpath, e.Name), src); err != nil {
+				_ = src.Close()
+				return err
+			}
+			if err := src.Close(); err != nil {
+				return err
+			}
+		case filemode.Dir:
+			if !recursive || e.Name == "testdata" {
+				continue
+			}
+			t2, err := r.TreeObject(e.Hash)
+			if err != nil {
+				return err
+			}
+			if err := addFiles(z, r, t2, path.Join(dirpath, e.Name), recursive); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+func writeZipFile(z *zip.Writer, pathname string, src io.Reader) (err error) {
+	defer derrors.Wrap(&err, "writeZipFile(zip, %q, src)", pathname)
+
+	dst, err := z.Create(pathname)
+	if err != nil {
+		return err
+	}
+	_, err = io.Copy(dst, src)
+	return err
+}
+
+// subTree looks non-recursively for a directory with the given name in t,
+// and returns the corresponding tree.
+// If a directory with such name doesn't exist in t, it returns os.ErrNotExist.
+func subTree(r *git.Repository, t *object.Tree, name string) (_ *object.Tree, err error) {
+	defer derrors.Wrap(&err, "subTree(repository, tree, %q)", name)
+
+	for _, e := range t.Entries {
+		if e.Name == name {
+			return r.TreeObject(e.Hash)
+		}
+	}
+	return nil, os.ErrNotExist
+}
+
+// Contains reports whether the given import path could be part of the Go standard library,
+// by reporting whether the first component lacks a '.'.
+func Contains(path string) bool {
+	if i := strings.IndexByte(path, '/'); i != -1 {
+		path = path[:i]
+	}
+	return !strings.Contains(path, ".")
+}
diff --git a/internal/stdlib/stdlib_test.go b/internal/stdlib/stdlib_test.go
new file mode 100644
index 0000000..0aef287
--- /dev/null
+++ b/internal/stdlib/stdlib_test.go
@@ -0,0 +1,396 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package stdlib
+
+import (
+	"errors"
+	"flag"
+	"io/fs"
+	"reflect"
+	"testing"
+
+	"golang.org/x/mod/semver"
+	"golang.org/x/pkgsite-metrics/internal/version"
+)
+
+var (
+	clone    = flag.Bool("clone", false, "test actual clones of the Go repo")
+	repoPath = flag.String("path", "", "path to Go repo to test")
+)
+
+func TestTagForVersion(t *testing.T) {
+	for _, test := range []struct {
+		name    string
+		version string
+		want    string
+		wantErr bool
+	}{
+		{
+			name:    "std version v1.0.0",
+			version: "v1.0.0",
+			want:    "go1",
+		},
+		{
+			name:    "std version v1.12.5",
+			version: "v1.12.5",
+			want:    "go1.12.5",
+		},
+		{
+			name:    "std version v1.13, incomplete canonical version",
+			version: "v1.13",
+			want:    "go1.13",
+		},
+		{
+			name:    "std version v1.13.0-beta.1",
+			version: "v1.13.0-beta.1",
+			want:    "go1.13beta1",
+		},
+		{
+			name:    "std version v1.9.0-rc.2",
+			version: "v1.9.0-rc.2",
+			want:    "go1.9rc2",
+		},
+		{
+			name:    "std with digitless prerelease",
+			version: "v1.13.0-prerelease",
+			want:    "go1.13prerelease",
+		},
+		{
+			name:    "version v1.13.0",
+			version: "v1.13.0",
+			want:    "go1.13",
+		},
+		{
+			name:    "master branch",
+			version: "master",
+			want:    "master",
+		},
+		{
+			name:    "bad std semver",
+			version: "v1.x",
+			wantErr: true,
+		},
+		{
+			name:    "more bad std semver",
+			version: "v1.0-",
+			wantErr: true,
+		},
+		{
+			name:    "bad prerelease",
+			version: "v1.13.0-beta1",
+			wantErr: true,
+		},
+		{
+			name:    "another bad prerelease",
+			version: "v1.13.0-whatevs99",
+			wantErr: true,
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			got, err := TagForVersion(test.version)
+			if (err != nil) != test.wantErr {
+				t.Errorf("TagForVersion(%q) = %q, %v, wantErr %v", test.version, got, err, test.wantErr)
+				return
+			}
+			if got != test.want {
+				t.Errorf("TagForVersion(%q) = %q, %v, wanted %q, %v", test.version, got, err, test.want, nil)
+			}
+		})
+	}
+}
+
+func TestMajorVersionForVersion(t *testing.T) {
+	for _, test := range []struct {
+		in   string
+		want string // empty => error
+	}{
+		{"", ""},
+		{"garbage", ""},
+		{"v1.0.0", "go1"},
+		{"v1.13.3", "go1"},
+		{"v1.9.0-rc.2", "go1"},
+		{"v2.1.3", "go2"},
+	} {
+		got, err := MajorVersionForVersion(test.in)
+		if (err != nil) != (test.want == "") {
+			t.Errorf("%q: err: got %v, wanted error: %t", test.in, err, test.want == "")
+		}
+		if err == nil && got != test.want {
+			t.Errorf("%q: got %q, want %q", test.in, got, test.want)
+		}
+	}
+}
+
+func TestContentDir(t *testing.T) {
+	defer WithTestData()()
+	for _, resolvedVersion := range []string{
+		"v1.3.2",
+		"v1.12.5",
+		"v1.14.6",
+		DevFuzz,
+		version.Master,
+	} {
+		t.Run(resolvedVersion, func(t *testing.T) {
+			cdir, gotResolvedVersion, gotTime, err := ContentDir(resolvedVersion)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if SupportedBranches[resolvedVersion] {
+				if !version.IsPseudo(gotResolvedVersion) {
+					t.Errorf("resolved version: %s is not a pseudo-version", gotResolvedVersion)
+				}
+			} else if gotResolvedVersion != resolvedVersion {
+				t.Errorf("resolved version: got %s, want %s", gotResolvedVersion, resolvedVersion)
+			}
+			if !gotTime.Equal(TestCommitTime) {
+				t.Errorf("commit time: got %s, want %s", gotTime, TestCommitTime)
+			}
+			checkContentDirFiles(t, cdir, resolvedVersion)
+		})
+	}
+}
+
+func TestContentDirCloneAndOpen(t *testing.T) {
+	run := func(t *testing.T) {
+		for _, resolvedVersion := range []string{
+			"v1.3.2",
+			"v1.14.6",
+			version.Master,
+			version.Latest,
+		} {
+			t.Run(resolvedVersion, func(t *testing.T) {
+				cdir, _, _, err := ContentDir(resolvedVersion)
+				if err != nil {
+					t.Fatal(err)
+				}
+				checkContentDirFiles(t, cdir, resolvedVersion)
+			})
+		}
+	}
+
+	t.Run("clone", func(t *testing.T) {
+		if !*clone {
+			t.Skip("-clone not supplied")
+		}
+		defer withGoRepo(&remoteGoRepo{})()
+		run(t)
+	})
+	t.Run("local", func(t *testing.T) {
+		if *repoPath == "" {
+			t.Skip("-path not supplied")
+		}
+		lgr, err := newLocalGoRepo(*repoPath)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer withGoRepo(lgr)()
+		run(t)
+	})
+}
+
+func checkContentDirFiles(t *testing.T, cdir fs.FS, resolvedVersion string) {
+	wantFiles := map[string]bool{
+		"LICENSE":               true,
+		"errors/errors.go":      true,
+		"errors/errors_test.go": true,
+	}
+	if semver.Compare(resolvedVersion, "v1.13.0") > 0 || resolvedVersion == TestMasterVersion {
+		wantFiles["cmd/README.vendor"] = true
+	}
+	if semver.Compare(resolvedVersion, "v1.14.0") > 0 {
+		wantFiles["context/context.go"] = true
+	}
+	const readmeVendorFile = "README.vendor"
+	if _, err := fs.Stat(cdir, readmeVendorFile); !errors.Is(err, fs.ErrNotExist) {
+		t.Fatalf("fs.Stat returned %v; want %q to be removed", err, readmeVendorFile)
+	}
+	err := fs.WalkDir(cdir, ".", func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if d.IsDir() {
+			return nil
+		}
+		delete(wantFiles, path)
+		return nil
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(wantFiles) > 0 {
+		t.Errorf("zip missing files: %v", reflect.ValueOf(wantFiles).MapKeys())
+	}
+}
+
+func TestZipInfo(t *testing.T) {
+	defer WithTestData()()
+
+	for _, tc := range []struct {
+		requestedVersion string
+		want             string
+	}{
+		{
+			requestedVersion: "latest",
+			want:             "v1.14.6",
+		},
+		{
+			requestedVersion: "master",
+			want:             "master",
+		},
+	} {
+		gotVersion, err := ZipInfo(tc.requestedVersion)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if want := tc.want; gotVersion != want {
+			t.Errorf("version: got %q, want %q", gotVersion, want)
+		}
+	}
+}
+
+func TestVersions(t *testing.T) {
+	testVersions := func(wants []string) {
+		got, err := Versions()
+		if err != nil {
+			t.Fatal(err)
+		}
+		gotmap := map[string]bool{}
+		for _, g := range got {
+			gotmap[g] = true
+		}
+		for _, w := range wants {
+			if !gotmap[w] {
+				t.Errorf("missing %s", w)
+			}
+		}
+	}
+
+	commonWants := []string{
+		"v1.4.2",
+		"v1.9.0-rc.1",
+		"v1.11.0",
+		"v1.13.0-beta.1",
+	}
+	otherWants := append([]string{"v1.17.6"}, commonWants...)
+	t.Run("test", func(t *testing.T) {
+		defer WithTestData()()
+		testVersions(commonWants)
+	})
+	t.Run("local", func(t *testing.T) {
+		if *repoPath == "" {
+			t.Skip("-path not supplied")
+		}
+		lgr, err := newLocalGoRepo(*repoPath)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer withGoRepo(lgr)()
+		testVersions(otherWants)
+	})
+	t.Run("remote", func(t *testing.T) {
+		if !*clone {
+			t.Skip("-clone not supplied")
+		}
+		defer withGoRepo(&remoteGoRepo{})()
+		testVersions(otherWants)
+	})
+}
+
+func TestVersionForTag(t *testing.T) {
+	for _, test := range []struct {
+		in, want string
+	}{
+		{"", ""},
+		{"go1", "v1.0.0"},
+		{"go1.9beta2", "v1.9.0-beta.2"},
+		{"go1.12", "v1.12.0"},
+		{"go1.9.7", "v1.9.7"},
+		{"go2.0", "v2.0.0"},
+		{"go1.9rc2", "v1.9.0-rc.2"},
+		{"go1.1beta", ""},
+		{"go1.0", ""},
+		{"weekly.2012-02-14", ""},
+		{"latest", "latest"},
+	} {
+		got := VersionForTag(test.in)
+		if got != test.want {
+			t.Errorf("VersionForTag(%q) = %q, want %q", test.in, got, test.want)
+		}
+	}
+}
+
+func TestContains(t *testing.T) {
+	for _, test := range []struct {
+		in   string
+		want bool
+	}{
+		{"fmt", true},
+		{"encoding/json", true},
+		{"something/with.dots", true},
+		{"example.com", false},
+		{"example.com/fmt", false},
+	} {
+		got := Contains(test.in)
+		if got != test.want {
+			t.Errorf("Contains(%q) = %t, want %t", test.in, got, test.want)
+		}
+	}
+}
+
+func TestDirectory(t *testing.T) {
+	for _, tc := range []struct {
+		version string
+		want    string
+	}{
+		{
+			version: "v1.3.0-beta2",
+			want:    "src/pkg",
+		},
+		{
+			version: "v1.16.0-beta1",
+			want:    "src",
+		},
+		{
+			version: "master",
+			want:    "src",
+		},
+	} {
+		got := Directory(tc.version)
+		if got != tc.want {
+			t.Errorf("Directory(%s) = %s, want %s", tc.version, got, tc.want)
+		}
+	}
+}
+
+func TestVersionMatchesHash(t *testing.T) {
+	v := "v0.0.0-20210910212848-c8dfa306babb"
+	h := "c8dfa306babb91e88f8ba25329b3ef8aa11944e1"
+	if !VersionMatchesHash(v, h) {
+		t.Error("got false, want true")
+	}
+	h = "c8dfa306babXb91e88f8ba25329b3ef8aa11944e1"
+	if VersionMatchesHash(v, h) {
+		t.Error("got true, want false")
+	}
+}
+
+func TestResolveSupportedBranches(t *testing.T) {
+	// Fix test
+	t.Skip()
+
+	got, err := ResolveSupportedBranches()
+	if err != nil {
+		t.Fatal(err)
+	}
+	// We can't check the hashes because they change, but we can check the keys.
+	for key := range got {
+		if !SupportedBranches[key] {
+			t.Errorf("got key %q not in SupportedBranches", key)
+		}
+	}
+	if g, w := len(got), len(SupportedBranches); g != w {
+		t.Errorf("got %d hashes, want %d", g, w)
+	}
+}
diff --git a/internal/stdlib/testdata/dev.fuzz/LICENSE b/internal/stdlib/testdata/dev.fuzz/LICENSE
new file mode 100644
index 0000000..6a66aea
--- /dev/null
+++ b/internal/stdlib/testdata/dev.fuzz/LICENSE
@@ -0,0 +1,27 @@
+Copyright (c) 2009 The Go Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+   * Redistributions of source code must retain the above copyright
+notice, this list of conditions and the following disclaimer.
+   * Redistributions in binary form must reproduce the above
+copyright notice, this list of conditions and the following disclaimer
+in the documentation and/or other materials provided with the
+distribution.
+   * Neither the name of Google Inc. nor the names of its
+contributors may be used to endorse or promote products derived from
+this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/internal/stdlib/testdata/dev.fuzz/README.md b/internal/stdlib/testdata/dev.fuzz/README.md
new file mode 100644
index 0000000..d6d2b9d
--- /dev/null
+++ b/internal/stdlib/testdata/dev.fuzz/README.md
@@ -0,0 +1 @@
+# The Go Programming Language
diff --git a/internal/stdlib/testdata/dev.fuzz/src/README.vendor b/internal/stdlib/testdata/dev.fuzz/src/README.vendor
new file mode 100644
index 0000000..e74fc2f
--- /dev/null
+++ b/internal/stdlib/testdata/dev.fuzz/src/README.vendor
@@ -0,0 +1,54 @@
+Vendoring in std and cmd
+========================
+
+The Go command maintains copies of external packages needed by the
+standard library in the src/vendor and src/cmd/vendor directories.
+
+In GOPATH mode, imports of vendored packages are resolved to these
+directories following normal vendor directory logic
+(see golang.org/s/go15vendor).
+
+In module mode, std and cmd are modules (defined in src/go.mod and
+src/cmd/go.mod). When a package outside std or cmd is imported
+by a package inside std or cmd, the import path is interpreted
+as if it had a "vendor/" prefix. For example, within "crypto/tls",
+an import of "golang.org/x/crypto/cryptobyte" resolves to
+"vendor/golang.org/x/crypto/cryptobyte". When a package with the
+same path is imported from a package outside std or cmd, it will
+be resolved normally. Consequently, a binary may be built with two
+copies of a package at different versions if the package is
+imported normally and vendored by the standard library.
+
+Vendored packages are internally renamed with a "vendor/" prefix
+to preserve the invariant that all packages have distinct paths.
+This is necessary to avoid compiler and linker conflicts. Adding
+a "vendor/" prefix also maintains the invariant that standard
+library packages begin with a dotless path element.
+
+The module requirements of std and cmd do not influence version
+selection in other modules. They are only considered when running
+module commands like 'go get' and 'go mod vendor' from a directory
+in GOROOT/src.
+
+Maintaining vendor directories
+==============================
+
+Before updating vendor directories, ensure that module mode is enabled.
+Make sure GO111MODULE=off is not set ('on' or 'auto' should work).
+
+Requirements may be added, updated, and removed with 'go get'.
+The vendor directory may be updated with 'go mod vendor'.
+A typical sequence might be:
+
+    cd src
+    go get -d golang.org/x/net@latest
+    go mod tidy
+    go mod vendor
+
+Use caution when passing '-u' to 'go get'. The '-u' flag updates
+modules providing all transitively imported packages, not only
+the module providing the target package.
+
+Note that 'go mod vendor' only copies packages that are transitively
+imported by packages in the current module. If a new package is needed,
+it should be imported before running 'go mod vendor'.
diff --git a/internal/stdlib/testdata/dev.fuzz/src/cmd/README.vendor b/internal/stdlib/testdata/dev.fuzz/src/cmd/README.vendor
new file mode 100644
index 0000000..ac0df5e
--- /dev/null
+++ b/internal/stdlib/testdata/dev.fuzz/src/cmd/README.vendor
@@ -0,0 +1,2 @@
+See src/README.vendor for information on loading vendored packages
+and updating the vendor directory.
diff --git a/internal/stdlib/testdata/dev.fuzz/src/errors/errors.go b/internal/stdlib/testdata/dev.fuzz/src/errors/errors.go
new file mode 100644
index 0000000..b8a4692
--- /dev/null
+++ b/internal/stdlib/testdata/dev.fuzz/src/errors/errors.go
@@ -0,0 +1,20 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package errors implements functions to manipulate errors.
+package errors
+
+// New returns an error that formats as the given text.
+func New(text string) error {
+	return &errorString{text}
+}
+
+// errorString is a trivial implementation of error.
+type errorString struct {
+	s string
+}
+
+func (e *errorString) Error() string {
+	return e.s
+}
diff --git a/internal/stdlib/testdata/dev.fuzz/src/errors/errors_test.go b/internal/stdlib/testdata/dev.fuzz/src/errors/errors_test.go
new file mode 100644
index 0000000..cf4df90
--- /dev/null
+++ b/internal/stdlib/testdata/dev.fuzz/src/errors/errors_test.go
@@ -0,0 +1,53 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errors_test
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+func TestNewEqual(t *testing.T) {
+	// Different allocations should not be equal.
+	if errors.New("abc") == errors.New("abc") {
+		t.Errorf(`New("abc") == New("abc")`)
+	}
+	if errors.New("abc") == errors.New("xyz") {
+		t.Errorf(`New("abc") == New("xyz")`)
+	}
+
+	// Same allocation should be equal to itself (not crash).
+	err := errors.New("jkl")
+	if err != err {
+		t.Errorf(`err != err`)
+	}
+}
+
+func TestErrorMethod(t *testing.T) {
+	err := errors.New("abc")
+	if err.Error() != "abc" {
+		t.Errorf(`New("abc").Error() = %q, want %q`, err.Error(), "abc")
+	}
+}
+
+func ExampleNew() {
+	err := errors.New("emit macho dwarf: elf header corrupted")
+	if err != nil {
+		fmt.Print(err)
+	}
+	// Output: emit macho dwarf: elf header corrupted
+}
+
+// The fmt package's Errorf function lets us use the package's formatting
+// features to create descriptive error messages.
+func ExampleNew_errorf() {
+	const name, id = "bimmler", 17
+	err := fmt.Errorf("user %q (id %d) not found", name, id)
+	if err != nil {
+		fmt.Print(err)
+	}
+	// Output: user "bimmler" (id 17) not found
+}
diff --git a/internal/stdlib/testdata/master/LICENSE b/internal/stdlib/testdata/master/LICENSE
new file mode 100644
index 0000000..6a66aea
--- /dev/null
+++ b/internal/stdlib/testdata/master/LICENSE
@@ -0,0 +1,27 @@
+Copyright (c) 2009 The Go Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+   * Redistributions of source code must retain the above copyright
+notice, this list of conditions and the following disclaimer.
+   * Redistributions in binary form must reproduce the above
+copyright notice, this list of conditions and the following disclaimer
+in the documentation and/or other materials provided with the
+distribution.
+   * Neither the name of Google Inc. nor the names of its
+contributors may be used to endorse or promote products derived from
+this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/internal/stdlib/testdata/master/README.md b/internal/stdlib/testdata/master/README.md
new file mode 100644
index 0000000..d6d2b9d
--- /dev/null
+++ b/internal/stdlib/testdata/master/README.md
@@ -0,0 +1 @@
+# The Go Programming Language
diff --git a/internal/stdlib/testdata/master/src/README.vendor b/internal/stdlib/testdata/master/src/README.vendor
new file mode 100644
index 0000000..e74fc2f
--- /dev/null
+++ b/internal/stdlib/testdata/master/src/README.vendor
@@ -0,0 +1,54 @@
+Vendoring in std and cmd
+========================
+
+The Go command maintains copies of external packages needed by the
+standard library in the src/vendor and src/cmd/vendor directories.
+
+In GOPATH mode, imports of vendored packages are resolved to these
+directories following normal vendor directory logic
+(see golang.org/s/go15vendor).
+
+In module mode, std and cmd are modules (defined in src/go.mod and
+src/cmd/go.mod). When a package outside std or cmd is imported
+by a package inside std or cmd, the import path is interpreted
+as if it had a "vendor/" prefix. For example, within "crypto/tls",
+an import of "golang.org/x/crypto/cryptobyte" resolves to
+"vendor/golang.org/x/crypto/cryptobyte". When a package with the
+same path is imported from a package outside std or cmd, it will
+be resolved normally. Consequently, a binary may be built with two
+copies of a package at different versions if the package is
+imported normally and vendored by the standard library.
+
+Vendored packages are internally renamed with a "vendor/" prefix
+to preserve the invariant that all packages have distinct paths.
+This is necessary to avoid compiler and linker conflicts. Adding
+a "vendor/" prefix also maintains the invariant that standard
+library packages begin with a dotless path element.
+
+The module requirements of std and cmd do not influence version
+selection in other modules. They are only considered when running
+module commands like 'go get' and 'go mod vendor' from a directory
+in GOROOT/src.
+
+Maintaining vendor directories
+==============================
+
+Before updating vendor directories, ensure that module mode is enabled.
+Make sure GO111MODULE=off is not set ('on' or 'auto' should work).
+
+Requirements may be added, updated, and removed with 'go get'.
+The vendor directory may be updated with 'go mod vendor'.
+A typical sequence might be:
+
+    cd src
+    go get -d golang.org/x/net@latest
+    go mod tidy
+    go mod vendor
+
+Use caution when passing '-u' to 'go get'. The '-u' flag updates
+modules providing all transitively imported packages, not only
+the module providing the target package.
+
+Note that 'go mod vendor' only copies packages that are transitively
+imported by packages in the current module. If a new package is needed,
+it should be imported before running 'go mod vendor'.
diff --git a/internal/stdlib/testdata/master/src/cmd/README.vendor b/internal/stdlib/testdata/master/src/cmd/README.vendor
new file mode 100644
index 0000000..ac0df5e
--- /dev/null
+++ b/internal/stdlib/testdata/master/src/cmd/README.vendor
@@ -0,0 +1,2 @@
+See src/README.vendor for information on loading vendored packages
+and updating the vendor directory.
diff --git a/internal/stdlib/testdata/master/src/errors/errors.go b/internal/stdlib/testdata/master/src/errors/errors.go
new file mode 100644
index 0000000..b8a4692
--- /dev/null
+++ b/internal/stdlib/testdata/master/src/errors/errors.go
@@ -0,0 +1,20 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package errors implements functions to manipulate errors.
+package errors
+
+// New returns an error that formats as the given text.
+func New(text string) error {
+	return &errorString{text}
+}
+
+// errorString is a trivial implementation of error.
+type errorString struct {
+	s string
+}
+
+func (e *errorString) Error() string {
+	return e.s
+}
diff --git a/internal/stdlib/testdata/master/src/errors/errors_test.go b/internal/stdlib/testdata/master/src/errors/errors_test.go
new file mode 100644
index 0000000..cf4df90
--- /dev/null
+++ b/internal/stdlib/testdata/master/src/errors/errors_test.go
@@ -0,0 +1,53 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errors_test
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+func TestNewEqual(t *testing.T) {
+	// Different allocations should not be equal.
+	if errors.New("abc") == errors.New("abc") {
+		t.Errorf(`New("abc") == New("abc")`)
+	}
+	if errors.New("abc") == errors.New("xyz") {
+		t.Errorf(`New("abc") == New("xyz")`)
+	}
+
+	// Same allocation should be equal to itself (not crash).
+	err := errors.New("jkl")
+	if err != err {
+		t.Errorf(`err != err`)
+	}
+}
+
+func TestErrorMethod(t *testing.T) {
+	err := errors.New("abc")
+	if err.Error() != "abc" {
+		t.Errorf(`New("abc").Error() = %q, want %q`, err.Error(), "abc")
+	}
+}
+
+func ExampleNew() {
+	err := errors.New("emit macho dwarf: elf header corrupted")
+	if err != nil {
+		fmt.Print(err)
+	}
+	// Output: emit macho dwarf: elf header corrupted
+}
+
+// The fmt package's Errorf function lets us use the package's formatting
+// features to create descriptive error messages.
+func ExampleNew_errorf() {
+	const name, id = "bimmler", 17
+	err := fmt.Errorf("user %q (id %d) not found", name, id)
+	if err != nil {
+		fmt.Print(err)
+	}
+	// Output: user "bimmler" (id 17) not found
+}
diff --git a/internal/stdlib/testdata/v1.12.5/LICENSE b/internal/stdlib/testdata/v1.12.5/LICENSE
new file mode 100644
index 0000000..6a66aea
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/LICENSE
@@ -0,0 +1,27 @@
+Copyright (c) 2009 The Go Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+   * Redistributions of source code must retain the above copyright
+notice, this list of conditions and the following disclaimer.
+   * Redistributions in binary form must reproduce the above
+copyright notice, this list of conditions and the following disclaimer
+in the documentation and/or other materials provided with the
+distribution.
+   * Neither the name of Google Inc. nor the names of its
+contributors may be used to endorse or promote products derived from
+this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/internal/stdlib/testdata/v1.12.5/README.md b/internal/stdlib/testdata/v1.12.5/README.md
new file mode 100644
index 0000000..d6d2b9d
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/README.md
@@ -0,0 +1 @@
+# The Go Programming Language
diff --git a/internal/stdlib/testdata/v1.12.5/src/_foo/README.md b/internal/stdlib/testdata/v1.12.5/src/_foo/README.md
new file mode 100644
index 0000000..177b08e
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/_foo/README.md
@@ -0,0 +1 @@
+Can not read content
diff --git a/internal/stdlib/testdata/v1.12.5/src/builtin/builtin.go b/internal/stdlib/testdata/v1.12.5/src/builtin/builtin.go
new file mode 100644
index 0000000..c78fe09
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/builtin/builtin.go
@@ -0,0 +1,263 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+	Package builtin provides documentation for Go's predeclared identifiers.
+	The items documented here are not actually in package builtin
+	but their descriptions here allow godoc to present documentation
+	for the language's special identifiers.
+*/
+package builtin
+
+// bool is the set of boolean values, true and false.
+type bool bool
+
+// true and false are the two untyped boolean values.
+const (
+	true  = 0 == 0 // Untyped bool.
+	false = 0 != 0 // Untyped bool.
+)
+
+// uint8 is the set of all unsigned 8-bit integers.
+// Range: 0 through 255.
+type uint8 uint8
+
+// uint16 is the set of all unsigned 16-bit integers.
+// Range: 0 through 65535.
+type uint16 uint16
+
+// uint32 is the set of all unsigned 32-bit integers.
+// Range: 0 through 4294967295.
+type uint32 uint32
+
+// uint64 is the set of all unsigned 64-bit integers.
+// Range: 0 through 18446744073709551615.
+type uint64 uint64
+
+// int8 is the set of all signed 8-bit integers.
+// Range: -128 through 127.
+type int8 int8
+
+// int16 is the set of all signed 16-bit integers.
+// Range: -32768 through 32767.
+type int16 int16
+
+// int32 is the set of all signed 32-bit integers.
+// Range: -2147483648 through 2147483647.
+type int32 int32
+
+// int64 is the set of all signed 64-bit integers.
+// Range: -9223372036854775808 through 9223372036854775807.
+type int64 int64
+
+// float32 is the set of all IEEE-754 32-bit floating-point numbers.
+type float32 float32
+
+// float64 is the set of all IEEE-754 64-bit floating-point numbers.
+type float64 float64
+
+// complex64 is the set of all complex numbers with float32 real and
+// imaginary parts.
+type complex64 complex64
+
+// complex128 is the set of all complex numbers with float64 real and
+// imaginary parts.
+type complex128 complex128
+
+// string is the set of all strings of 8-bit bytes, conventionally but not
+// necessarily representing UTF-8-encoded text. A string may be empty, but
+// not nil. Values of string type are immutable.
+type string string
+
+// int is a signed integer type that is at least 32 bits in size. It is a
+// distinct type, however, and not an alias for, say, int32.
+type int int
+
+// uint is an unsigned integer type that is at least 32 bits in size. It is a
+// distinct type, however, and not an alias for, say, uint32.
+type uint uint
+
+// uintptr is an integer type that is large enough to hold the bit pattern of
+// any pointer.
+type uintptr uintptr
+
+// byte is an alias for uint8 and is equivalent to uint8 in all ways. It is
+// used, by convention, to distinguish byte values from 8-bit unsigned
+// integer values.
+type byte = uint8
+
+// rune is an alias for int32 and is equivalent to int32 in all ways. It is
+// used, by convention, to distinguish character values from integer values.
+type rune = int32
+
+// iota is a predeclared identifier representing the untyped integer ordinal
+// number of the current const specification in a (usually parenthesized)
+// const declaration. It is zero-indexed.
+const iota = 0 // Untyped int.
+
+// nil is a predeclared identifier representing the zero value for a
+// pointer, channel, func, interface, map, or slice type.
+var nil Type // Type must be a pointer, channel, func, interface, map, or slice type
+
+// Type is here for the purposes of documentation only. It is a stand-in
+// for any Go type, but represents the same type for any given function
+// invocation.
+type Type int
+
+// Type1 is here for the purposes of documentation only. It is a stand-in
+// for any Go type, but represents the same type for any given function
+// invocation.
+type Type1 int
+
+// IntegerType is here for the purposes of documentation only. It is a stand-in
+// for any integer type: int, uint, int8 etc.
+type IntegerType int
+
+// FloatType is here for the purposes of documentation only. It is a stand-in
+// for either float type: float32 or float64.
+type FloatType float32
+
+// ComplexType is here for the purposes of documentation only. It is a
+// stand-in for either complex type: complex64 or complex128.
+type ComplexType complex64
+
+// The append built-in function appends elements to the end of a slice. If
+// it has sufficient capacity, the destination is resliced to accommodate the
+// new elements. If it does not, a new underlying array will be allocated.
+// Append returns the updated slice. It is therefore necessary to store the
+// result of append, often in the variable holding the slice itself:
+//	slice = append(slice, elem1, elem2)
+//	slice = append(slice, anotherSlice...)
+// As a special case, it is legal to append a string to a byte slice, like this:
+//	slice = append([]byte("hello "), "world"...)
+func append(slice []Type, elems ...Type) []Type
+
+// The copy built-in function copies elements from a source slice into a
+// destination slice. (As a special case, it also will copy bytes from a
+// string to a slice of bytes.) The source and destination may overlap. Copy
+// returns the number of elements copied, which will be the minimum of
+// len(src) and len(dst).
+func copy(dst, src []Type) int
+
+// The delete built-in function deletes the element with the specified key
+// (m[key]) from the map. If m is nil or there is no such element, delete
+// is a no-op.
+func delete(m map[Type]Type1, key Type)
+
+// The len built-in function returns the length of v, according to its type:
+//	Array: the number of elements in v.
+//	Pointer to array: the number of elements in *v (even if v is nil).
+//	Slice, or map: the number of elements in v; if v is nil, len(v) is zero.
+//	String: the number of bytes in v.
+//	Channel: the number of elements queued (unread) in the channel buffer;
+//	if v is nil, len(v) is zero.
+// For some arguments, such as a string literal or a simple array expression, the
+// result can be a constant. See the Go language specification's "Length and
+// capacity" section for details.
+func len(v Type) int
+
+// The cap built-in function returns the capacity of v, according to its type:
+//	Array: the number of elements in v (same as len(v)).
+//	Pointer to array: the number of elements in *v (same as len(v)).
+//	Slice: the maximum length the slice can reach when resliced;
+//	if v is nil, cap(v) is zero.
+//	Channel: the channel buffer capacity, in units of elements;
+//	if v is nil, cap(v) is zero.
+// For some arguments, such as a simple array expression, the result can be a
+// constant. See the Go language specification's "Length and capacity" section for
+// details.
+func cap(v Type) int
+
+// The make built-in function allocates and initializes an object of type
+// slice, map, or chan (only). Like new, the first argument is a type, not a
+// value. Unlike new, make's return type is the same as the type of its
+// argument, not a pointer to it. The specification of the result depends on
+// the type:
+//	Slice: The size specifies the length. The capacity of the slice is
+//	equal to its length. A second integer argument may be provided to
+//	specify a different capacity; it must be no smaller than the
+//	length. For example, make([]int, 0, 10) allocates an underlying array
+//	of size 10 and returns a slice of length 0 and capacity 10 that is
+//	backed by this underlying array.
+//	Map: An empty map is allocated with enough space to hold the
+//	specified number of elements. The size may be omitted, in which case
+//	a small starting size is allocated.
+//	Channel: The channel's buffer is initialized with the specified
+//	buffer capacity. If zero, or the size is omitted, the channel is
+//	unbuffered.
+func make(t Type, size ...IntegerType) Type
+
+// The new built-in function allocates memory. The first argument is a type,
+// not a value, and the value returned is a pointer to a newly
+// allocated zero value of that type.
+func new(Type) *Type
+
+// The complex built-in function constructs a complex value from two
+// floating-point values. The real and imaginary parts must be of the same
+// size, either float32 or float64 (or assignable to them), and the return
+// value will be the corresponding complex type (complex64 for float32,
+// complex128 for float64).
+func complex(r, i FloatType) ComplexType
+
+// The real built-in function returns the real part of the complex number c.
+// The return value will be floating point type corresponding to the type of c.
+func real(c ComplexType) FloatType
+
+// The imag built-in function returns the imaginary part of the complex
+// number c. The return value will be floating point type corresponding to
+// the type of c.
+func imag(c ComplexType) FloatType
+
+// The close built-in function closes a channel, which must be either
+// bidirectional or send-only. It should be executed only by the sender,
+// never the receiver, and has the effect of shutting down the channel after
+// the last sent value is received. After the last value has been received
+// from a closed channel c, any receive from c will succeed without
+// blocking, returning the zero value for the channel element. The form
+//	x, ok := <-c
+// will also set ok to false for a closed channel.
+func close(c chan<- Type)
+
+// The panic built-in function stops normal execution of the current
+// goroutine. When a function F calls panic, normal execution of F stops
+// immediately. Any functions whose execution was deferred by F are run in
+// the usual way, and then F returns to its caller. To the caller G, the
+// invocation of F then behaves like a call to panic, terminating G's
+// execution and running any deferred functions. This continues until all
+// functions in the executing goroutine have stopped, in reverse order. At
+// that point, the program is terminated and the error condition is reported,
+// including the value of the argument to panic. This termination sequence
+// is called panicking and can be controlled by the built-in function
+// recover.
+func panic(v interface{})
+
+// The recover built-in function allows a program to manage behavior of a
+// panicking goroutine. Executing a call to recover inside a deferred
+// function (but not any function called by it) stops the panicking sequence
+// by restoring normal execution and retrieves the error value passed to the
+// call of panic. If recover is called outside the deferred function it will
+// not stop a panicking sequence. In this case, or when the goroutine is not
+// panicking, or if the argument supplied to panic was nil, recover returns
+// nil. Thus the return value from recover reports whether the goroutine is
+// panicking.
+func recover() interface{}
+
+// The print built-in function formats its arguments in an
+// implementation-specific way and writes the result to standard error.
+// Print is useful for bootstrapping and debugging; it is not guaranteed
+// to stay in the language.
+func print(args ...Type)
+
+// The println built-in function formats its arguments in an
+// implementation-specific way and writes the result to standard error.
+// Spaces are always added between arguments and a newline is appended.
+// Println is useful for bootstrapping and debugging; it is not guaranteed
+// to stay in the language.
+func println(args ...Type)
+
+// The error built-in interface type is the conventional interface for
+// representing an error condition, with the nil value representing no error.
+type error interface {
+	Error() string
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/cmd/pprof/README b/internal/stdlib/testdata/v1.12.5/src/cmd/pprof/README
new file mode 100644
index 0000000..9de1f51
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/cmd/pprof/README
@@ -0,0 +1 @@
+This directory is the copy of Google's pprof shipped as part of the Go distribution.
diff --git a/internal/stdlib/testdata/v1.12.5/src/cmd/pprof/doc.go b/internal/stdlib/testdata/v1.12.5/src/cmd/pprof/doc.go
new file mode 100644
index 0000000..84de036
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/cmd/pprof/doc.go
@@ -0,0 +1,12 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Pprof interprets and displays profiles of Go programs.
+//
+// Usage:
+//
+//	go tool pprof binary profile
+//
+// For more information, see https://blog.golang.org/profiling-go-programs.
+package main
diff --git a/internal/stdlib/testdata/v1.12.5/src/cmd/pprof/pprof.go b/internal/stdlib/testdata/v1.12.5/src/cmd/pprof/pprof.go
new file mode 100644
index 0000000..89f3969
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/cmd/pprof/pprof.go
@@ -0,0 +1,376 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// pprof is a tool for visualization of profile.data. It is based on
+// the upstream version at github.com/google/pprof, with minor
+// modifications specific to the Go distribution. Please consider
+// upstreaming any modifications to these packages.
+
+package main
+
+import (
+	"crypto/tls"
+	"debug/dwarf"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"os"
+	"regexp"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+
+	"cmd/internal/objfile"
+
+	"github.com/google/pprof/driver"
+	"github.com/google/pprof/profile"
+)
+
+func main() {
+	options := &driver.Options{
+		Fetch: new(fetcher),
+		Obj:   new(objTool),
+		UI:    newUI(),
+	}
+	if err := driver.PProf(options); err != nil {
+		fmt.Fprintf(os.Stderr, "%v\n", err)
+		os.Exit(2)
+	}
+}
+
+type fetcher struct {
+}
+
+func (f *fetcher) Fetch(src string, duration, timeout time.Duration) (*profile.Profile, string, error) {
+	sourceURL, timeout := adjustURL(src, duration, timeout)
+	if sourceURL == "" {
+		// Could not recognize URL, let regular pprof attempt to fetch the profile (eg. from a file)
+		return nil, "", nil
+	}
+	fmt.Fprintln(os.Stderr, "Fetching profile over HTTP from", sourceURL)
+	if duration > 0 {
+		fmt.Fprintf(os.Stderr, "Please wait... (%v)\n", duration)
+	}
+	p, err := getProfile(sourceURL, timeout)
+	return p, sourceURL, err
+}
+
+func getProfile(source string, timeout time.Duration) (*profile.Profile, error) {
+	url, err := url.Parse(source)
+	if err != nil {
+		return nil, err
+	}
+
+	var tlsConfig *tls.Config
+	if url.Scheme == "https+insecure" {
+		tlsConfig = &tls.Config{
+			InsecureSkipVerify: true,
+		}
+		url.Scheme = "https"
+		source = url.String()
+	}
+
+	client := &http.Client{
+		Transport: &http.Transport{
+			ResponseHeaderTimeout: timeout + 5*time.Second,
+			Proxy:                 http.ProxyFromEnvironment,
+			TLSClientConfig:       tlsConfig,
+		},
+	}
+	resp, err := client.Get(source)
+	if err != nil {
+		return nil, err
+	}
+	if resp.StatusCode != http.StatusOK {
+		defer resp.Body.Close()
+		return nil, statusCodeError(resp)
+	}
+	return profile.Parse(resp.Body)
+}
+
+func statusCodeError(resp *http.Response) error {
+	if resp.Header.Get("X-Go-Pprof") != "" && strings.Contains(resp.Header.Get("Content-Type"), "text/plain") {
+		// error is from pprof endpoint
+		if body, err := io.ReadAll(resp.Body); err == nil {
+			return fmt.Errorf("server response: %s - %s", resp.Status, body)
+		}
+	}
+	return fmt.Errorf("server response: %s", resp.Status)
+}
+
+// cpuProfileHandler is the Go pprof CPU profile handler URL.
+const cpuProfileHandler = "/debug/pprof/profile"
+
+// adjustURL applies the duration/timeout values and Go specific defaults
+func adjustURL(source string, duration, timeout time.Duration) (string, time.Duration) {
+	u, err := url.Parse(source)
+	if err != nil || (u.Host == "" && u.Scheme != "" && u.Scheme != "file") {
+		// Try adding http:// to catch sources of the form hostname:port/path.
+		// url.Parse treats "hostname" as the scheme.
+		u, err = url.Parse("http://" + source)
+	}
+	if err != nil || u.Host == "" {
+		return "", 0
+	}
+
+	if u.Path == "" || u.Path == "/" {
+		u.Path = cpuProfileHandler
+	}
+
+	// Apply duration/timeout overrides to URL.
+	values := u.Query()
+	if duration > 0 {
+		values.Set("seconds", fmt.Sprint(int(duration.Seconds())))
+	} else {
+		if urlSeconds := values.Get("seconds"); urlSeconds != "" {
+			if us, err := strconv.ParseInt(urlSeconds, 10, 32); err == nil {
+				duration = time.Duration(us) * time.Second
+			}
+		}
+	}
+	if timeout <= 0 {
+		if duration > 0 {
+			timeout = duration + duration/2
+		} else {
+			timeout = 60 * time.Second
+		}
+	}
+	u.RawQuery = values.Encode()
+	return u.String(), timeout
+}
+
+// objTool implements driver.ObjTool using Go libraries
+// (instead of invoking GNU binutils).
+type objTool struct {
+	mu          sync.Mutex
+	disasmCache map[string]*objfile.Disasm
+}
+
+func (*objTool) Open(name string, start, limit, offset uint64) (driver.ObjFile, error) {
+	of, err := objfile.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	f := &file{
+		name: name,
+		file: of,
+	}
+	if start != 0 {
+		if load, err := of.LoadAddress(); err == nil {
+			f.offset = start - load
+		}
+	}
+	return f, nil
+}
+
+func (*objTool) Demangle(names []string) (map[string]string, error) {
+	// No C++, nothing to demangle.
+	return make(map[string]string), nil
+}
+
+func (t *objTool) Disasm(file string, start, end uint64) ([]driver.Inst, error) {
+	d, err := t.cachedDisasm(file)
+	if err != nil {
+		return nil, err
+	}
+	var asm []driver.Inst
+	d.Decode(start, end, nil, func(pc, size uint64, file string, line int, text string) {
+		asm = append(asm, driver.Inst{Addr: pc, File: file, Line: line, Text: text})
+	})
+	return asm, nil
+}
+
+func (t *objTool) cachedDisasm(file string) (*objfile.Disasm, error) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	if t.disasmCache == nil {
+		t.disasmCache = make(map[string]*objfile.Disasm)
+	}
+	d := t.disasmCache[file]
+	if d != nil {
+		return d, nil
+	}
+	f, err := objfile.Open(file)
+	if err != nil {
+		return nil, err
+	}
+	d, err = f.Disasm()
+	f.Close()
+	if err != nil {
+		return nil, err
+	}
+	t.disasmCache[file] = d
+	return d, nil
+}
+
+func (*objTool) SetConfig(config string) {
+	// config is usually used to say what binaries to invoke.
+	// Ignore entirely.
+}
+
+// file implements driver.ObjFile using Go libraries
+// (instead of invoking GNU binutils).
+// A file represents a single executable being analyzed.
+type file struct {
+	name   string
+	offset uint64
+	sym    []objfile.Sym
+	file   *objfile.File
+	pcln   objfile.Liner
+
+	triedDwarf bool
+	dwarf      *dwarf.Data
+}
+
+func (f *file) Name() string {
+	return f.name
+}
+
+func (f *file) Base() uint64 {
+	// No support for shared libraries.
+	return 0
+}
+
+func (f *file) BuildID() string {
+	// No support for build ID.
+	return ""
+}
+
+func (f *file) SourceLine(addr uint64) ([]driver.Frame, error) {
+	if f.pcln == nil {
+		pcln, err := f.file.PCLineTable()
+		if err != nil {
+			return nil, err
+		}
+		f.pcln = pcln
+	}
+	addr -= f.offset
+	file, line, fn := f.pcln.PCToLine(addr)
+	if fn != nil {
+		frame := []driver.Frame{
+			{
+				Func: fn.Name,
+				File: file,
+				Line: line,
+			},
+		}
+		return frame, nil
+	}
+
+	frames := f.dwarfSourceLine(addr)
+	if frames != nil {
+		return frames, nil
+	}
+
+	return nil, fmt.Errorf("no line information for PC=%#x", addr)
+}
+
+// dwarfSourceLine tries to get file/line information using DWARF.
+// This is for C functions that appear in the profile.
+// Returns nil if there is no information available.
+func (f *file) dwarfSourceLine(addr uint64) []driver.Frame {
+	if f.dwarf == nil && !f.triedDwarf {
+		// Ignore any error--we don't care exactly why there
+		// is no DWARF info.
+		f.dwarf, _ = f.file.DWARF()
+		f.triedDwarf = true
+	}
+
+	if f.dwarf != nil {
+		r := f.dwarf.Reader()
+		unit, err := r.SeekPC(addr)
+		if err == nil {
+			if frames := f.dwarfSourceLineEntry(r, unit, addr); frames != nil {
+				return frames
+			}
+		}
+	}
+
+	return nil
+}
+
+// dwarfSourceLineEntry tries to get file/line information from a
+// DWARF compilation unit. Returns nil if it doesn't find anything.
+func (f *file) dwarfSourceLineEntry(r *dwarf.Reader, entry *dwarf.Entry, addr uint64) []driver.Frame {
+	lines, err := f.dwarf.LineReader(entry)
+	if err != nil {
+		return nil
+	}
+	var lentry dwarf.LineEntry
+	if err := lines.SeekPC(addr, &lentry); err != nil {
+		return nil
+	}
+
+	// Try to find the function name.
+	name := ""
+FindName:
+	for entry, err := r.Next(); entry != nil && err == nil; entry, err = r.Next() {
+		if entry.Tag == dwarf.TagSubprogram {
+			ranges, err := f.dwarf.Ranges(entry)
+			if err != nil {
+				return nil
+			}
+			for _, pcs := range ranges {
+				if pcs[0] <= addr && addr < pcs[1] {
+					var ok bool
+					// TODO: AT_linkage_name, AT_MIPS_linkage_name.
+					name, ok = entry.Val(dwarf.AttrName).(string)
+					if ok {
+						break FindName
+					}
+				}
+			}
+		}
+	}
+
+	// TODO: Report inlined functions.
+
+	frames := []driver.Frame{
+		{
+			Func: name,
+			File: lentry.File.Name,
+			Line: lentry.Line,
+		},
+	}
+
+	return frames
+}
+
+func (f *file) Symbols(r *regexp.Regexp, addr uint64) ([]*driver.Sym, error) {
+	if f.sym == nil {
+		sym, err := f.file.Symbols()
+		if err != nil {
+			return nil, err
+		}
+		f.sym = sym
+	}
+	var out []*driver.Sym
+	for _, s := range f.sym {
+		// Ignore a symbol with address 0 and size 0.
+		// An ELF STT_FILE symbol will look like that.
+		if s.Addr == 0 && s.Size == 0 {
+			continue
+		}
+		if (r == nil || r.MatchString(s.Name)) && (addr == 0 || s.Addr <= addr && addr < s.Addr+uint64(s.Size)) {
+			out = append(out, &driver.Sym{
+				Name:  []string{s.Name},
+				File:  f.name,
+				Start: s.Addr,
+				End:   s.Addr + uint64(s.Size) - 1,
+			})
+		}
+	}
+	return out, nil
+}
+
+func (f *file) Close() error {
+	f.file.Close()
+	return nil
+}
+
+// newUI will be set in readlineui.go in some platforms
+// for interactive readline functionality.
+var newUI = func() driver.UI { return nil }
diff --git a/internal/stdlib/testdata/v1.12.5/src/cmd/pprof/readlineui.go b/internal/stdlib/testdata/v1.12.5/src/cmd/pprof/readlineui.go
new file mode 100644
index 0000000..5b9701a
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/cmd/pprof/readlineui.go
@@ -0,0 +1,120 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains an driver.UI implementation
+// that provides the readline functionality if possible.
+
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris windows
+// +build !appengine
+// +build !android
+
+package main
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"strings"
+
+	"github.com/google/pprof/driver"
+	"golang.org/x/crypto/ssh/terminal"
+)
+
+func init() {
+	newUI = newReadlineUI
+}
+
+// readlineUI implements driver.UI interface using the
+// golang.org/x/crypto/ssh/terminal package.
+// The upstream pprof command implements the same functionality
+// using the github.com/chzyer/readline package.
+type readlineUI struct {
+	term *terminal.Terminal
+}
+
+func newReadlineUI() driver.UI {
+	// disable readline UI in dumb terminal. (golang.org/issue/26254)
+	if v := strings.ToLower(os.Getenv("TERM")); v == "" || v == "dumb" {
+		return nil
+	}
+	// test if we can use terminal.ReadLine
+	// that assumes operation in the raw mode.
+	oldState, err := terminal.MakeRaw(0)
+	if err != nil {
+		return nil
+	}
+	terminal.Restore(0, oldState)
+
+	rw := struct {
+		io.Reader
+		io.Writer
+	}{os.Stdin, os.Stderr}
+	return &readlineUI{term: terminal.NewTerminal(rw, "")}
+}
+
+// Read returns a line of text (a command) read from the user.
+// prompt is printed before reading the command.
+func (r *readlineUI) ReadLine(prompt string) (string, error) {
+	r.term.SetPrompt(prompt)
+
+	// skip error checking because we tested it
+	// when creating this readlineUI initially.
+	oldState, _ := terminal.MakeRaw(0)
+	defer terminal.Restore(0, oldState)
+
+	s, err := r.term.ReadLine()
+	return s, err
+}
+
+// Print shows a message to the user.
+// It formats the text as fmt.Print would and adds a final \n if not already present.
+// For line-based UI, Print writes to standard error.
+// (Standard output is reserved for report data.)
+func (r *readlineUI) Print(args ...interface{}) {
+	r.print(false, args...)
+}
+
+// PrintErr shows an error message to the user.
+// It formats the text as fmt.Print would and adds a final \n if not already present.
+// For line-based UI, PrintErr writes to standard error.
+func (r *readlineUI) PrintErr(args ...interface{}) {
+	r.print(true, args...)
+}
+
+func (r *readlineUI) print(withColor bool, args ...interface{}) {
+	text := fmt.Sprint(args...)
+	if !strings.HasSuffix(text, "\n") {
+		text += "\n"
+	}
+	if withColor {
+		text = colorize(text)
+	}
+	fmt.Fprint(r.term, text)
+}
+
+// colorize prints the msg in red using ANSI color escapes.
+func colorize(msg string) string {
+	const red = 31
+	var colorEscape = fmt.Sprintf("\033[0;%dm", red)
+	var colorResetEscape = "\033[0m"
+	return colorEscape + msg + colorResetEscape
+}
+
+// IsTerminal reports whether the UI is known to be tied to an
+// interactive terminal (as opposed to being redirected to a file).
+func (r *readlineUI) IsTerminal() bool {
+	const stdout = 1
+	return terminal.IsTerminal(stdout)
+}
+
+// WantBrowser indicates whether browser should be opened with the -http option.
+func (r *readlineUI) WantBrowser() bool {
+	return r.IsTerminal()
+}
+
+// SetAutoComplete instructs the UI to call complete(cmd) to obtain
+// the auto-completion of cmd, if the UI supports auto-completion at all.
+func (r *readlineUI) SetAutoComplete(complete func(string) string) {
+	// TODO: Implement auto-completion support.
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/context/benchmark_test.go b/internal/stdlib/testdata/v1.12.5/src/context/benchmark_test.go
new file mode 100644
index 0000000..5d56863
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/context/benchmark_test.go
@@ -0,0 +1,140 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package context_test
+
+import (
+	. "context"
+	"fmt"
+	"runtime"
+	"sync"
+	"testing"
+	"time"
+)
+
+func BenchmarkCommonParentCancel(b *testing.B) {
+	root := WithValue(Background(), "key", "value")
+	shared, sharedcancel := WithCancel(root)
+	defer sharedcancel()
+
+	b.ResetTimer()
+	b.RunParallel(func(pb *testing.PB) {
+		x := 0
+		for pb.Next() {
+			ctx, cancel := WithCancel(shared)
+			if ctx.Value("key").(string) != "value" {
+				b.Fatal("should not be reached")
+			}
+			for i := 0; i < 100; i++ {
+				x /= x + 1
+			}
+			cancel()
+			for i := 0; i < 100; i++ {
+				x /= x + 1
+			}
+		}
+	})
+}
+
+func BenchmarkWithTimeout(b *testing.B) {
+	for concurrency := 40; concurrency <= 4e5; concurrency *= 100 {
+		name := fmt.Sprintf("concurrency=%d", concurrency)
+		b.Run(name, func(b *testing.B) {
+			benchmarkWithTimeout(b, concurrency)
+		})
+	}
+}
+
+func benchmarkWithTimeout(b *testing.B, concurrentContexts int) {
+	gomaxprocs := runtime.GOMAXPROCS(0)
+	perPContexts := concurrentContexts / gomaxprocs
+	root := Background()
+
+	// Generate concurrent contexts.
+	var wg sync.WaitGroup
+	ccf := make([][]CancelFunc, gomaxprocs)
+	for i := range ccf {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			cf := make([]CancelFunc, perPContexts)
+			for j := range cf {
+				_, cf[j] = WithTimeout(root, time.Hour)
+			}
+			ccf[i] = cf
+		}(i)
+	}
+	wg.Wait()
+
+	b.ResetTimer()
+	b.RunParallel(func(pb *testing.PB) {
+		wcf := make([]CancelFunc, 10)
+		for pb.Next() {
+			for i := range wcf {
+				_, wcf[i] = WithTimeout(root, time.Hour)
+			}
+			for _, f := range wcf {
+				f()
+			}
+		}
+	})
+	b.StopTimer()
+
+	for _, cf := range ccf {
+		for _, f := range cf {
+			f()
+		}
+	}
+}
+
+func BenchmarkCancelTree(b *testing.B) {
+	depths := []int{1, 10, 100, 1000}
+	for _, d := range depths {
+		b.Run(fmt.Sprintf("depth=%d", d), func(b *testing.B) {
+			b.Run("Root=Background", func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					buildContextTree(Background(), d)
+				}
+			})
+			b.Run("Root=OpenCanceler", func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					ctx, cancel := WithCancel(Background())
+					buildContextTree(ctx, d)
+					cancel()
+				}
+			})
+			b.Run("Root=ClosedCanceler", func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					ctx, cancel := WithCancel(Background())
+					cancel()
+					buildContextTree(ctx, d)
+				}
+			})
+		})
+	}
+}
+
+func buildContextTree(root Context, depth int) {
+	for d := 0; d < depth; d++ {
+		root, _ = WithCancel(root)
+	}
+}
+
+func BenchmarkCheckCanceled(b *testing.B) {
+	ctx, cancel := WithCancel(Background())
+	cancel()
+	b.Run("Err", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			ctx.Err()
+		}
+	})
+	b.Run("Done", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			select {
+			case <-ctx.Done():
+			default:
+			}
+		}
+	})
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/context/context.go b/internal/stdlib/testdata/v1.12.5/src/context/context.go
new file mode 100644
index 0000000..21a40d5
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/context/context.go
@@ -0,0 +1,493 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package context defines the Context type, which carries deadlines,
+// cancelation signals, and other request-scoped values across API boundaries
+// and between processes.
+//
+// Incoming requests to a server should create a Context, and outgoing
+// calls to servers should accept a Context. The chain of function
+// calls between them must propagate the Context, optionally replacing
+// it with a derived Context created using WithCancel, WithDeadline,
+// WithTimeout, or WithValue. When a Context is canceled, all
+// Contexts derived from it are also canceled.
+//
+// The WithCancel, WithDeadline, and WithTimeout functions take a
+// Context (the parent) and return a derived Context (the child) and a
+// CancelFunc. Calling the CancelFunc cancels the child and its
+// children, removes the parent's reference to the child, and stops
+// any associated timers. Failing to call the CancelFunc leaks the
+// child and its children until the parent is canceled or the timer
+// fires. The go vet tool checks that CancelFuncs are used on all
+// control-flow paths.
+//
+// Programs that use Contexts should follow these rules to keep interfaces
+// consistent across packages and enable static analysis tools to check context
+// propagation:
+//
+// Do not store Contexts inside a struct type; instead, pass a Context
+// explicitly to each function that needs it. The Context should be the first
+// parameter, typically named ctx:
+//
+// 	func DoSomething(ctx context.Context, arg Arg) error {
+// 		// ... use ctx ...
+// 	}
+//
+// Do not pass a nil Context, even if a function permits it. Pass context.TODO
+// if you are unsure about which Context to use.
+//
+// Use context Values only for request-scoped data that transits processes and
+// APIs, not for passing optional parameters to functions.
+//
+// The same Context may be passed to functions running in different goroutines;
+// Contexts are safe for simultaneous use by multiple goroutines.
+//
+// See https://blog.golang.org/context for example code for a server that uses
+// Contexts.
+package context
+
+import (
+	"errors"
+	"fmt"
+	"reflect"
+	"sync"
+	"time"
+)
+
+// A Context carries a deadline, a cancelation signal, and other values across
+// API boundaries.
+//
+// Context's methods may be called by multiple goroutines simultaneously.
+type Context interface {
+	// Deadline returns the time when work done on behalf of this context
+	// should be canceled. Deadline returns ok==false when no deadline is
+	// set. Successive calls to Deadline return the same results.
+	Deadline() (deadline time.Time, ok bool)
+
+	// Done returns a channel that's closed when work done on behalf of this
+	// context should be canceled. Done may return nil if this context can
+	// never be canceled. Successive calls to Done return the same value.
+	//
+	// WithCancel arranges for Done to be closed when cancel is called;
+	// WithDeadline arranges for Done to be closed when the deadline
+	// expires; WithTimeout arranges for Done to be closed when the timeout
+	// elapses.
+	//
+	// Done is provided for use in select statements:
+	//
+	//  // Stream generates values with DoSomething and sends them to out
+	//  // until DoSomething returns an error or ctx.Done is closed.
+	//  func Stream(ctx context.Context, out chan<- Value) error {
+	//  	for {
+	//  		v, err := DoSomething(ctx)
+	//  		if err != nil {
+	//  			return err
+	//  		}
+	//  		select {
+	//  		case <-ctx.Done():
+	//  			return ctx.Err()
+	//  		case out <- v:
+	//  		}
+	//  	}
+	//  }
+	//
+	// See https://blog.golang.org/pipelines for more examples of how to use
+	// a Done channel for cancelation.
+	Done() <-chan struct{}
+
+	// If Done is not yet closed, Err returns nil.
+	// If Done is closed, Err returns a non-nil error explaining why:
+	// Canceled if the context was canceled
+	// or DeadlineExceeded if the context's deadline passed.
+	// After Err returns a non-nil error, successive calls to Err return the same error.
+	Err() error
+
+	// Value returns the value associated with this context for key, or nil
+	// if no value is associated with key. Successive calls to Value with
+	// the same key returns the same result.
+	//
+	// Use context values only for request-scoped data that transits
+	// processes and API boundaries, not for passing optional parameters to
+	// functions.
+	//
+	// A key identifies a specific value in a Context. Functions that wish
+	// to store values in Context typically allocate a key in a global
+	// variable then use that key as the argument to context.WithValue and
+	// Context.Value. A key can be any type that supports equality;
+	// packages should define keys as an unexported type to avoid
+	// collisions.
+	//
+	// Packages that define a Context key should provide type-safe accessors
+	// for the values stored using that key:
+	//
+	// 	// Package user defines a User type that's stored in Contexts.
+	// 	package user
+	//
+	// 	import "context"
+	//
+	// 	// User is the type of value stored in the Contexts.
+	// 	type User struct {...}
+	//
+	// 	// key is an unexported type for keys defined in this package.
+	// 	// This prevents collisions with keys defined in other packages.
+	// 	type key int
+	//
+	// 	// userKey is the key for user.User values in Contexts. It is
+	// 	// unexported; clients use user.NewContext and user.FromContext
+	// 	// instead of using this key directly.
+	// 	var userKey key
+	//
+	// 	// NewContext returns a new Context that carries value u.
+	// 	func NewContext(ctx context.Context, u *User) context.Context {
+	// 		return context.WithValue(ctx, userKey, u)
+	// 	}
+	//
+	// 	// FromContext returns the User value stored in ctx, if any.
+	// 	func FromContext(ctx context.Context) (*User, bool) {
+	// 		u, ok := ctx.Value(userKey).(*User)
+	// 		return u, ok
+	// 	}
+	Value(key interface{}) interface{}
+}
+
+// Canceled is the error returned by Context.Err when the context is canceled.
+var Canceled = errors.New("context canceled")
+
+// DeadlineExceeded is the error returned by Context.Err when the context's
+// deadline passes.
+var DeadlineExceeded error = deadlineExceededError{}
+
+type deadlineExceededError struct{}
+
+func (deadlineExceededError) Error() string   { return "context deadline exceeded" }
+func (deadlineExceededError) Timeout() bool   { return true }
+func (deadlineExceededError) Temporary() bool { return true }
+
+// An emptyCtx is never canceled, has no values, and has no deadline. It is not
+// struct{}, since vars of this type must have distinct addresses.
+type emptyCtx int
+
+func (*emptyCtx) Deadline() (deadline time.Time, ok bool) {
+	return
+}
+
+func (*emptyCtx) Done() <-chan struct{} {
+	return nil
+}
+
+func (*emptyCtx) Err() error {
+	return nil
+}
+
+func (*emptyCtx) Value(key interface{}) interface{} {
+	return nil
+}
+
+func (e *emptyCtx) String() string {
+	switch e {
+	case background:
+		return "context.Background"
+	case todo:
+		return "context.TODO"
+	}
+	return "unknown empty Context"
+}
+
+var (
+	background = new(emptyCtx)
+	todo       = new(emptyCtx)
+)
+
+// Background returns a non-nil, empty Context. It is never canceled, has no
+// values, and has no deadline. It is typically used by the main function,
+// initialization, and tests, and as the top-level Context for incoming
+// requests.
+func Background() Context {
+	return background
+}
+
+// TODO returns a non-nil, empty Context. Code should use context.TODO when
+// it's unclear which Context to use or it is not yet available (because the
+// surrounding function has not yet been extended to accept a Context
+// parameter).
+func TODO() Context {
+	return todo
+}
+
+// A CancelFunc tells an operation to abandon its work.
+// A CancelFunc does not wait for the work to stop.
+// After the first call, subsequent calls to a CancelFunc do nothing.
+type CancelFunc func()
+
+// WithCancel returns a copy of parent with a new Done channel. The returned
+// context's Done channel is closed when the returned cancel function is called
+// or when the parent context's Done channel is closed, whichever happens first.
+//
+// Canceling this context releases resources associated with it, so code should
+// call cancel as soon as the operations running in this Context complete.
+func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
+	c := newCancelCtx(parent)
+	propagateCancel(parent, &c)
+	return &c, func() { c.cancel(true, Canceled) }
+}
+
+// newCancelCtx returns an initialized cancelCtx.
+func newCancelCtx(parent Context) cancelCtx {
+	return cancelCtx{Context: parent}
+}
+
+// propagateCancel arranges for child to be canceled when parent is.
+func propagateCancel(parent Context, child canceler) {
+	if parent.Done() == nil {
+		return // parent is never canceled
+	}
+	if p, ok := parentCancelCtx(parent); ok {
+		p.mu.Lock()
+		if p.err != nil {
+			// parent has already been canceled
+			child.cancel(false, p.err)
+		} else {
+			if p.children == nil {
+				p.children = make(map[canceler]struct{})
+			}
+			p.children[child] = struct{}{}
+		}
+		p.mu.Unlock()
+	} else {
+		go func() {
+			select {
+			case <-parent.Done():
+				child.cancel(false, parent.Err())
+			case <-child.Done():
+			}
+		}()
+	}
+}
+
+// parentCancelCtx follows a chain of parent references until it finds a
+// *cancelCtx. This function understands how each of the concrete types in this
+// package represents its parent.
+func parentCancelCtx(parent Context) (*cancelCtx, bool) {
+	for {
+		switch c := parent.(type) {
+		case *cancelCtx:
+			return c, true
+		case *timerCtx:
+			return &c.cancelCtx, true
+		case *valueCtx:
+			parent = c.Context
+		default:
+			return nil, false
+		}
+	}
+}
+
+// removeChild removes a context from its parent.
+func removeChild(parent Context, child canceler) {
+	p, ok := parentCancelCtx(parent)
+	if !ok {
+		return
+	}
+	p.mu.Lock()
+	if p.children != nil {
+		delete(p.children, child)
+	}
+	p.mu.Unlock()
+}
+
+// A canceler is a context type that can be canceled directly. The
+// implementations are *cancelCtx and *timerCtx.
+type canceler interface {
+	cancel(removeFromParent bool, err error)
+	Done() <-chan struct{}
+}
+
+// closedchan is a reusable closed channel.
+var closedchan = make(chan struct{})
+
+func init() {
+	close(closedchan)
+}
+
+// A cancelCtx can be canceled. When canceled, it also cancels any children
+// that implement canceler.
+type cancelCtx struct {
+	Context
+
+	mu       sync.Mutex            // protects following fields
+	done     chan struct{}         // created lazily, closed by first cancel call
+	children map[canceler]struct{} // set to nil by the first cancel call
+	err      error                 // set to non-nil by the first cancel call
+}
+
+func (c *cancelCtx) Done() <-chan struct{} {
+	c.mu.Lock()
+	if c.done == nil {
+		c.done = make(chan struct{})
+	}
+	d := c.done
+	c.mu.Unlock()
+	return d
+}
+
+func (c *cancelCtx) Err() error {
+	c.mu.Lock()
+	err := c.err
+	c.mu.Unlock()
+	return err
+}
+
+func (c *cancelCtx) String() string {
+	return fmt.Sprintf("%v.WithCancel", c.Context)
+}
+
+// cancel closes c.done, cancels each of c's children, and, if
+// removeFromParent is true, removes c from its parent's children.
+func (c *cancelCtx) cancel(removeFromParent bool, err error) {
+	if err == nil {
+		panic("context: internal error: missing cancel error")
+	}
+	c.mu.Lock()
+	if c.err != nil {
+		c.mu.Unlock()
+		return // already canceled
+	}
+	c.err = err
+	if c.done == nil {
+		c.done = closedchan
+	} else {
+		close(c.done)
+	}
+	for child := range c.children {
+		// NOTE: acquiring the child's lock while holding parent's lock.
+		child.cancel(false, err)
+	}
+	c.children = nil
+	c.mu.Unlock()
+
+	if removeFromParent {
+		removeChild(c.Context, c)
+	}
+}
+
+// WithDeadline returns a copy of the parent context with the deadline adjusted
+// to be no later than d. If the parent's deadline is already earlier than d,
+// WithDeadline(parent, d) is semantically equivalent to parent. The returned
+// context's Done channel is closed when the deadline expires, when the returned
+// cancel function is called, or when the parent context's Done channel is
+// closed, whichever happens first.
+//
+// Canceling this context releases resources associated with it, so code should
+// call cancel as soon as the operations running in this Context complete.
+func WithDeadline(parent Context, d time.Time) (Context, CancelFunc) {
+	if cur, ok := parent.Deadline(); ok && cur.Before(d) {
+		// The current deadline is already sooner than the new one.
+		return WithCancel(parent)
+	}
+	c := &timerCtx{
+		cancelCtx: newCancelCtx(parent),
+		deadline:  d,
+	}
+	propagateCancel(parent, c)
+	dur := time.Until(d)
+	if dur <= 0 {
+		c.cancel(true, DeadlineExceeded) // deadline has already passed
+		return c, func() { c.cancel(false, Canceled) }
+	}
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.err == nil {
+		c.timer = time.AfterFunc(dur, func() {
+			c.cancel(true, DeadlineExceeded)
+		})
+	}
+	return c, func() { c.cancel(true, Canceled) }
+}
+
+// A timerCtx carries a timer and a deadline. It embeds a cancelCtx to
+// implement Done and Err. It implements cancel by stopping its timer then
+// delegating to cancelCtx.cancel.
+type timerCtx struct {
+	cancelCtx
+	timer *time.Timer // Under cancelCtx.mu.
+
+	deadline time.Time
+}
+
+func (c *timerCtx) Deadline() (deadline time.Time, ok bool) {
+	return c.deadline, true
+}
+
+func (c *timerCtx) String() string {
+	return fmt.Sprintf("%v.WithDeadline(%s [%s])", c.cancelCtx.Context, c.deadline, time.Until(c.deadline))
+}
+
+func (c *timerCtx) cancel(removeFromParent bool, err error) {
+	c.cancelCtx.cancel(false, err)
+	if removeFromParent {
+		// Remove this timerCtx from its parent cancelCtx's children.
+		removeChild(c.cancelCtx.Context, c)
+	}
+	c.mu.Lock()
+	if c.timer != nil {
+		c.timer.Stop()
+		c.timer = nil
+	}
+	c.mu.Unlock()
+}
+
+// WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)).
+//
+// Canceling this context releases resources associated with it, so code should
+// call cancel as soon as the operations running in this Context complete:
+//
+// 	func slowOperationWithTimeout(ctx context.Context) (Result, error) {
+// 		ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)
+// 		defer cancel()  // releases resources if slowOperation completes before timeout elapses
+// 		return slowOperation(ctx)
+// 	}
+func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
+	return WithDeadline(parent, time.Now().Add(timeout))
+}
+
+// WithValue returns a copy of parent in which the value associated with key is
+// val.
+//
+// Use context Values only for request-scoped data that transits processes and
+// APIs, not for passing optional parameters to functions.
+//
+// The provided key must be comparable and should not be of type
+// string or any other built-in type to avoid collisions between
+// packages using context. Users of WithValue should define their own
+// types for keys. To avoid allocating when assigning to an
+// interface{}, context keys often have concrete type
+// struct{}. Alternatively, exported context key variables' static
+// type should be a pointer or interface.
+func WithValue(parent Context, key, val interface{}) Context {
+	if key == nil {
+		panic("nil key")
+	}
+	if !reflect.TypeOf(key).Comparable() {
+		panic("key is not comparable")
+	}
+	return &valueCtx{parent, key, val}
+}
+
+// A valueCtx carries a key-value pair. It implements Value for that key and
+// delegates all other calls to the embedded Context.
+type valueCtx struct {
+	Context
+	key, val interface{}
+}
+
+func (c *valueCtx) String() string {
+	return fmt.Sprintf("%v.WithValue(%#v, %#v)", c.Context, c.key, c.val)
+}
+
+func (c *valueCtx) Value(key interface{}) interface{} {
+	if c.key == key {
+		return c.val
+	}
+	return c.Context.Value(key)
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/context/context_test.go b/internal/stdlib/testdata/v1.12.5/src/context/context_test.go
new file mode 100644
index 0000000..0b6ca74
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/context/context_test.go
@@ -0,0 +1,650 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package context
+
+import (
+	"fmt"
+	"math/rand"
+	"runtime"
+	"strings"
+	"sync"
+	"time"
+)
+
+type testingT interface {
+	Error(args ...interface{})
+	Errorf(format string, args ...interface{})
+	Fail()
+	FailNow()
+	Failed() bool
+	Fatal(args ...interface{})
+	Fatalf(format string, args ...interface{})
+	Log(args ...interface{})
+	Logf(format string, args ...interface{})
+	Name() string
+	Skip(args ...interface{})
+	SkipNow()
+	Skipf(format string, args ...interface{})
+	Skipped() bool
+}
+
+// otherContext is a Context that's not one of the types defined in context.go.
+// This lets us test code paths that differ based on the underlying type of the
+// Context.
+type otherContext struct {
+	Context
+}
+
+func XTestBackground(t testingT) {
+	c := Background()
+	if c == nil {
+		t.Fatalf("Background returned nil")
+	}
+	select {
+	case x := <-c.Done():
+		t.Errorf("<-c.Done() == %v want nothing (it should block)", x)
+	default:
+	}
+	if got, want := fmt.Sprint(c), "context.Background"; got != want {
+		t.Errorf("Background().String() = %q want %q", got, want)
+	}
+}
+
+func XTestTODO(t testingT) {
+	c := TODO()
+	if c == nil {
+		t.Fatalf("TODO returned nil")
+	}
+	select {
+	case x := <-c.Done():
+		t.Errorf("<-c.Done() == %v want nothing (it should block)", x)
+	default:
+	}
+	if got, want := fmt.Sprint(c), "context.TODO"; got != want {
+		t.Errorf("TODO().String() = %q want %q", got, want)
+	}
+}
+
+func XTestWithCancel(t testingT) {
+	c1, cancel := WithCancel(Background())
+
+	if got, want := fmt.Sprint(c1), "context.Background.WithCancel"; got != want {
+		t.Errorf("c1.String() = %q want %q", got, want)
+	}
+
+	o := otherContext{c1}
+	c2, _ := WithCancel(o)
+	contexts := []Context{c1, o, c2}
+
+	for i, c := range contexts {
+		if d := c.Done(); d == nil {
+			t.Errorf("c[%d].Done() == %v want non-nil", i, d)
+		}
+		if e := c.Err(); e != nil {
+			t.Errorf("c[%d].Err() == %v want nil", i, e)
+		}
+
+		select {
+		case x := <-c.Done():
+			t.Errorf("<-c.Done() == %v want nothing (it should block)", x)
+		default:
+		}
+	}
+
+	cancel()
+	time.Sleep(100 * time.Millisecond) // let cancelation propagate
+
+	for i, c := range contexts {
+		select {
+		case <-c.Done():
+		default:
+			t.Errorf("<-c[%d].Done() blocked, but shouldn't have", i)
+		}
+		if e := c.Err(); e != Canceled {
+			t.Errorf("c[%d].Err() == %v want %v", i, e, Canceled)
+		}
+	}
+}
+
+func contains(m map[canceler]struct{}, key canceler) bool {
+	_, ret := m[key]
+	return ret
+}
+
+func XTestParentFinishesChild(t testingT) {
+	// Context tree:
+	// parent -> cancelChild
+	// parent -> valueChild -> timerChild
+	parent, cancel := WithCancel(Background())
+	cancelChild, stop := WithCancel(parent)
+	defer stop()
+	valueChild := WithValue(parent, "key", "value")
+	timerChild, stop := WithTimeout(valueChild, 10000*time.Hour)
+	defer stop()
+
+	select {
+	case x := <-parent.Done():
+		t.Errorf("<-parent.Done() == %v want nothing (it should block)", x)
+	case x := <-cancelChild.Done():
+		t.Errorf("<-cancelChild.Done() == %v want nothing (it should block)", x)
+	case x := <-timerChild.Done():
+		t.Errorf("<-timerChild.Done() == %v want nothing (it should block)", x)
+	case x := <-valueChild.Done():
+		t.Errorf("<-valueChild.Done() == %v want nothing (it should block)", x)
+	default:
+	}
+
+	// The parent's children should contain the two cancelable children.
+	pc := parent.(*cancelCtx)
+	cc := cancelChild.(*cancelCtx)
+	test := timerChild.(*timerCtx)
+	pc.mu.Lock()
+	if len(pc.children) != 2 || !contains(pc.children, cc) || !contains(pc.children, test) {
+		t.Errorf("bad linkage: pc.children = %v, want %v and %v",
+			pc.children, cc, test)
+	}
+	pc.mu.Unlock()
+
+	if p, ok := parentCancelCtx(cc.Context); !ok || p != pc {
+		t.Errorf("bad linkage: parentCancelCtx(cancelChild.Context) = %v, %v want %v, true", p, ok, pc)
+	}
+	if p, ok := parentCancelCtx(test.Context); !ok || p != pc {
+		t.Errorf("bad linkage: parentCancelCtx(timerChild.Context) = %v, %v want %v, true", p, ok, pc)
+	}
+
+	cancel()
+
+	pc.mu.Lock()
+	if len(pc.children) != 0 {
+		t.Errorf("pc.cancel didn't clear pc.children = %v", pc.children)
+	}
+	pc.mu.Unlock()
+
+	// parent and children should all be finished.
+	check := func(ctx Context, name string) {
+		select {
+		case <-ctx.Done():
+		default:
+			t.Errorf("<-%s.Done() blocked, but shouldn't have", name)
+		}
+		if e := ctx.Err(); e != Canceled {
+			t.Errorf("%s.Err() == %v want %v", name, e, Canceled)
+		}
+	}
+	check(parent, "parent")
+	check(cancelChild, "cancelChild")
+	check(valueChild, "valueChild")
+	check(timerChild, "timerChild")
+
+	// WithCancel should return a canceled context on a canceled parent.
+	precanceledChild := WithValue(parent, "key", "value")
+	select {
+	case <-precanceledChild.Done():
+	default:
+		t.Errorf("<-precanceledChild.Done() blocked, but shouldn't have")
+	}
+	if e := precanceledChild.Err(); e != Canceled {
+		t.Errorf("precanceledChild.Err() == %v want %v", e, Canceled)
+	}
+}
+
+func XTestChildFinishesFirst(t testingT) {
+	cancelable, stop := WithCancel(Background())
+	defer stop()
+	for _, parent := range []Context{Background(), cancelable} {
+		child, cancel := WithCancel(parent)
+
+		select {
+		case x := <-parent.Done():
+			t.Errorf("<-parent.Done() == %v want nothing (it should block)", x)
+		case x := <-child.Done():
+			t.Errorf("<-child.Done() == %v want nothing (it should block)", x)
+		default:
+		}
+
+		cc := child.(*cancelCtx)
+		pc, pcok := parent.(*cancelCtx) // pcok == false when parent == Background()
+		if p, ok := parentCancelCtx(cc.Context); ok != pcok || (ok && pc != p) {
+			t.Errorf("bad linkage: parentCancelCtx(cc.Context) = %v, %v want %v, %v", p, ok, pc, pcok)
+		}
+
+		if pcok {
+			pc.mu.Lock()
+			if len(pc.children) != 1 || !contains(pc.children, cc) {
+				t.Errorf("bad linkage: pc.children = %v, cc = %v", pc.children, cc)
+			}
+			pc.mu.Unlock()
+		}
+
+		cancel()
+
+		if pcok {
+			pc.mu.Lock()
+			if len(pc.children) != 0 {
+				t.Errorf("child's cancel didn't remove self from pc.children = %v", pc.children)
+			}
+			pc.mu.Unlock()
+		}
+
+		// child should be finished.
+		select {
+		case <-child.Done():
+		default:
+			t.Errorf("<-child.Done() blocked, but shouldn't have")
+		}
+		if e := child.Err(); e != Canceled {
+			t.Errorf("child.Err() == %v want %v", e, Canceled)
+		}
+
+		// parent should not be finished.
+		select {
+		case x := <-parent.Done():
+			t.Errorf("<-parent.Done() == %v want nothing (it should block)", x)
+		default:
+		}
+		if e := parent.Err(); e != nil {
+			t.Errorf("parent.Err() == %v want nil", e)
+		}
+	}
+}
+
+func testDeadline(c Context, name string, failAfter time.Duration, t testingT) {
+	select {
+	case <-time.After(failAfter):
+		t.Fatalf("%s: context should have timed out", name)
+	case <-c.Done():
+	}
+	if e := c.Err(); e != DeadlineExceeded {
+		t.Errorf("%s: c.Err() == %v; want %v", name, e, DeadlineExceeded)
+	}
+}
+
+func XTestDeadline(t testingT) {
+	c, _ := WithDeadline(Background(), time.Now().Add(50*time.Millisecond))
+	if got, prefix := fmt.Sprint(c), "context.Background.WithDeadline("; !strings.HasPrefix(got, prefix) {
+		t.Errorf("c.String() = %q want prefix %q", got, prefix)
+	}
+	testDeadline(c, "WithDeadline", time.Second, t)
+
+	c, _ = WithDeadline(Background(), time.Now().Add(50*time.Millisecond))
+	o := otherContext{c}
+	testDeadline(o, "WithDeadline+otherContext", time.Second, t)
+
+	c, _ = WithDeadline(Background(), time.Now().Add(50*time.Millisecond))
+	o = otherContext{c}
+	c, _ = WithDeadline(o, time.Now().Add(4*time.Second))
+	testDeadline(c, "WithDeadline+otherContext+WithDeadline", 2*time.Second, t)
+
+	c, _ = WithDeadline(Background(), time.Now().Add(-time.Millisecond))
+	testDeadline(c, "WithDeadline+inthepast", time.Second, t)
+
+	c, _ = WithDeadline(Background(), time.Now())
+	testDeadline(c, "WithDeadline+now", time.Second, t)
+}
+
+func XTestTimeout(t testingT) {
+	c, _ := WithTimeout(Background(), 50*time.Millisecond)
+	if got, prefix := fmt.Sprint(c), "context.Background.WithDeadline("; !strings.HasPrefix(got, prefix) {
+		t.Errorf("c.String() = %q want prefix %q", got, prefix)
+	}
+	testDeadline(c, "WithTimeout", time.Second, t)
+
+	c, _ = WithTimeout(Background(), 50*time.Millisecond)
+	o := otherContext{c}
+	testDeadline(o, "WithTimeout+otherContext", time.Second, t)
+
+	c, _ = WithTimeout(Background(), 50*time.Millisecond)
+	o = otherContext{c}
+	c, _ = WithTimeout(o, 3*time.Second)
+	testDeadline(c, "WithTimeout+otherContext+WithTimeout", 2*time.Second, t)
+}
+
+func XTestCanceledTimeout(t testingT) {
+	c, _ := WithTimeout(Background(), time.Second)
+	o := otherContext{c}
+	c, cancel := WithTimeout(o, 2*time.Second)
+	cancel()
+	time.Sleep(100 * time.Millisecond) // let cancelation propagate
+	select {
+	case <-c.Done():
+	default:
+		t.Errorf("<-c.Done() blocked, but shouldn't have")
+	}
+	if e := c.Err(); e != Canceled {
+		t.Errorf("c.Err() == %v want %v", e, Canceled)
+	}
+}
+
+type key1 int
+type key2 int
+
+var k1 = key1(1)
+var k2 = key2(1) // same int as k1, different type
+var k3 = key2(3) // same type as k2, different int
+
+func XTestValues(t testingT) {
+	check := func(c Context, nm, v1, v2, v3 string) {
+		if v, ok := c.Value(k1).(string); ok == (len(v1) == 0) || v != v1 {
+			t.Errorf(`%s.Value(k1).(string) = %q, %t want %q, %t`, nm, v, ok, v1, len(v1) != 0)
+		}
+		if v, ok := c.Value(k2).(string); ok == (len(v2) == 0) || v != v2 {
+			t.Errorf(`%s.Value(k2).(string) = %q, %t want %q, %t`, nm, v, ok, v2, len(v2) != 0)
+		}
+		if v, ok := c.Value(k3).(string); ok == (len(v3) == 0) || v != v3 {
+			t.Errorf(`%s.Value(k3).(string) = %q, %t want %q, %t`, nm, v, ok, v3, len(v3) != 0)
+		}
+	}
+
+	c0 := Background()
+	check(c0, "c0", "", "", "")
+
+	c1 := WithValue(Background(), k1, "c1k1")
+	check(c1, "c1", "c1k1", "", "")
+
+	if got, want := fmt.Sprint(c1), `context.Background.WithValue(1, "c1k1")`; got != want {
+		t.Errorf("c.String() = %q want %q", got, want)
+	}
+
+	c2 := WithValue(c1, k2, "c2k2")
+	check(c2, "c2", "c1k1", "c2k2", "")
+
+	c3 := WithValue(c2, k3, "c3k3")
+	check(c3, "c2", "c1k1", "c2k2", "c3k3")
+
+	c4 := WithValue(c3, k1, nil)
+	check(c4, "c4", "", "c2k2", "c3k3")
+
+	o0 := otherContext{Background()}
+	check(o0, "o0", "", "", "")
+
+	o1 := otherContext{WithValue(Background(), k1, "c1k1")}
+	check(o1, "o1", "c1k1", "", "")
+
+	o2 := WithValue(o1, k2, "o2k2")
+	check(o2, "o2", "c1k1", "o2k2", "")
+
+	o3 := otherContext{c4}
+	check(o3, "o3", "", "c2k2", "c3k3")
+
+	o4 := WithValue(o3, k3, nil)
+	check(o4, "o4", "", "c2k2", "")
+}
+
+func XTestAllocs(t testingT, testingShort func() bool, testingAllocsPerRun func(int, func()) float64) {
+	bg := Background()
+	for _, test := range []struct {
+		desc       string
+		f          func()
+		limit      float64
+		gccgoLimit float64
+	}{
+		{
+			desc:       "Background()",
+			f:          func() { Background() },
+			limit:      0,
+			gccgoLimit: 0,
+		},
+		{
+			desc: fmt.Sprintf("WithValue(bg, %v, nil)", k1),
+			f: func() {
+				c := WithValue(bg, k1, nil)
+				c.Value(k1)
+			},
+			limit:      3,
+			gccgoLimit: 3,
+		},
+		{
+			desc: "WithTimeout(bg, 15*time.Millisecond)",
+			f: func() {
+				c, _ := WithTimeout(bg, 15*time.Millisecond)
+				<-c.Done()
+			},
+			limit:      8,
+			gccgoLimit: 15,
+		},
+		{
+			desc: "WithCancel(bg)",
+			f: func() {
+				c, cancel := WithCancel(bg)
+				cancel()
+				<-c.Done()
+			},
+			limit:      5,
+			gccgoLimit: 8,
+		},
+		{
+			desc: "WithTimeout(bg, 5*time.Millisecond)",
+			f: func() {
+				c, cancel := WithTimeout(bg, 5*time.Millisecond)
+				cancel()
+				<-c.Done()
+			},
+			limit:      8,
+			gccgoLimit: 25,
+		},
+	} {
+		limit := test.limit
+		if runtime.Compiler == "gccgo" {
+			// gccgo does not yet do escape analysis.
+			// TODO(iant): Remove this when gccgo does do escape analysis.
+			limit = test.gccgoLimit
+		}
+		numRuns := 100
+		if testingShort() {
+			numRuns = 10
+		}
+		if n := testingAllocsPerRun(numRuns, test.f); n > limit {
+			t.Errorf("%s allocs = %f want %d", test.desc, n, int(limit))
+		}
+	}
+}
+
+func XTestSimultaneousCancels(t testingT) {
+	root, cancel := WithCancel(Background())
+	m := map[Context]CancelFunc{root: cancel}
+	q := []Context{root}
+	// Create a tree of contexts.
+	for len(q) != 0 && len(m) < 100 {
+		parent := q[0]
+		q = q[1:]
+		for i := 0; i < 4; i++ {
+			ctx, cancel := WithCancel(parent)
+			m[ctx] = cancel
+			q = append(q, ctx)
+		}
+	}
+	// Start all the cancels in a random order.
+	var wg sync.WaitGroup
+	wg.Add(len(m))
+	for _, cancel := range m {
+		go func(cancel CancelFunc) {
+			cancel()
+			wg.Done()
+		}(cancel)
+	}
+	// Wait on all the contexts in a random order.
+	for ctx := range m {
+		select {
+		case <-ctx.Done():
+		case <-time.After(1 * time.Second):
+			buf := make([]byte, 10<<10)
+			n := runtime.Stack(buf, true)
+			t.Fatalf("timed out waiting for <-ctx.Done(); stacks:\n%s", buf[:n])
+		}
+	}
+	// Wait for all the cancel functions to return.
+	done := make(chan struct{})
+	go func() {
+		wg.Wait()
+		close(done)
+	}()
+	select {
+	case <-done:
+	case <-time.After(1 * time.Second):
+		buf := make([]byte, 10<<10)
+		n := runtime.Stack(buf, true)
+		t.Fatalf("timed out waiting for cancel functions; stacks:\n%s", buf[:n])
+	}
+}
+
+func XTestInterlockedCancels(t testingT) {
+	parent, cancelParent := WithCancel(Background())
+	child, cancelChild := WithCancel(parent)
+	go func() {
+		parent.Done()
+		cancelChild()
+	}()
+	cancelParent()
+	select {
+	case <-child.Done():
+	case <-time.After(1 * time.Second):
+		buf := make([]byte, 10<<10)
+		n := runtime.Stack(buf, true)
+		t.Fatalf("timed out waiting for child.Done(); stacks:\n%s", buf[:n])
+	}
+}
+
+func XTestLayersCancel(t testingT) {
+	testLayers(t, time.Now().UnixNano(), false)
+}
+
+func XTestLayersTimeout(t testingT) {
+	testLayers(t, time.Now().UnixNano(), true)
+}
+
+func testLayers(t testingT, seed int64, testTimeout bool) {
+	rand.Seed(seed)
+	errorf := func(format string, a ...interface{}) {
+		t.Errorf(fmt.Sprintf("seed=%d: %s", seed, format), a...)
+	}
+	const (
+		timeout   = 200 * time.Millisecond
+		minLayers = 30
+	)
+	type value int
+	var (
+		vals      []*value
+		cancels   []CancelFunc
+		numTimers int
+		ctx       = Background()
+	)
+	for i := 0; i < minLayers || numTimers == 0 || len(cancels) == 0 || len(vals) == 0; i++ {
+		switch rand.Intn(3) {
+		case 0:
+			v := new(value)
+			ctx = WithValue(ctx, v, v)
+			vals = append(vals, v)
+		case 1:
+			var cancel CancelFunc
+			ctx, cancel = WithCancel(ctx)
+			cancels = append(cancels, cancel)
+		case 2:
+			var cancel CancelFunc
+			ctx, cancel = WithTimeout(ctx, timeout)
+			cancels = append(cancels, cancel)
+			numTimers++
+		}
+	}
+	checkValues := func(when string) {
+		for _, key := range vals {
+			if val := ctx.Value(key).(*value); key != val {
+				errorf("%s: ctx.Value(%p) = %p want %p", when, key, val, key)
+			}
+		}
+	}
+	select {
+	case <-ctx.Done():
+		errorf("ctx should not be canceled yet")
+	default:
+	}
+	if s, prefix := fmt.Sprint(ctx), "context.Background."; !strings.HasPrefix(s, prefix) {
+		t.Errorf("ctx.String() = %q want prefix %q", s, prefix)
+	}
+	t.Log(ctx)
+	checkValues("before cancel")
+	if testTimeout {
+		select {
+		case <-ctx.Done():
+		case <-time.After(timeout + time.Second):
+			errorf("ctx should have timed out")
+		}
+		checkValues("after timeout")
+	} else {
+		cancel := cancels[rand.Intn(len(cancels))]
+		cancel()
+		select {
+		case <-ctx.Done():
+		default:
+			errorf("ctx should be canceled")
+		}
+		checkValues("after cancel")
+	}
+}
+
+func XTestCancelRemoves(t testingT) {
+	checkChildren := func(when string, ctx Context, want int) {
+		if got := len(ctx.(*cancelCtx).children); got != want {
+			t.Errorf("%s: context has %d children, want %d", when, got, want)
+		}
+	}
+
+	ctx, _ := WithCancel(Background())
+	checkChildren("after creation", ctx, 0)
+	_, cancel := WithCancel(ctx)
+	checkChildren("with WithCancel child ", ctx, 1)
+	cancel()
+	checkChildren("after canceling WithCancel child", ctx, 0)
+
+	ctx, _ = WithCancel(Background())
+	checkChildren("after creation", ctx, 0)
+	_, cancel = WithTimeout(ctx, 60*time.Minute)
+	checkChildren("with WithTimeout child ", ctx, 1)
+	cancel()
+	checkChildren("after canceling WithTimeout child", ctx, 0)
+}
+
+func XTestWithCancelCanceledParent(t testingT) {
+	parent, pcancel := WithCancel(Background())
+	pcancel()
+
+	c, _ := WithCancel(parent)
+	select {
+	case <-c.Done():
+	case <-time.After(5 * time.Second):
+		t.Fatal("timeout waiting for Done")
+	}
+	if got, want := c.Err(), Canceled; got != want {
+		t.Errorf("child not cancelled; got = %v, want = %v", got, want)
+	}
+}
+
+func XTestWithValueChecksKey(t testingT) {
+	panicVal := recoveredValue(func() { WithValue(Background(), []byte("foo"), "bar") })
+	if panicVal == nil {
+		t.Error("expected panic")
+	}
+	panicVal = recoveredValue(func() { WithValue(Background(), nil, "bar") })
+	if got, want := fmt.Sprint(panicVal), "nil key"; got != want {
+		t.Errorf("panic = %q; want %q", got, want)
+	}
+}
+
+func recoveredValue(fn func()) (v interface{}) {
+	defer func() { v = recover() }()
+	fn()
+	return
+}
+
+func XTestDeadlineExceededSupportsTimeout(t testingT) {
+	i, ok := DeadlineExceeded.(interface {
+		Timeout() bool
+	})
+	if !ok {
+		t.Fatal("DeadlineExceeded does not support Timeout interface")
+	}
+	if !i.Timeout() {
+		t.Fatal("wrong value for timeout")
+	}
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/context/example_test.go b/internal/stdlib/testdata/v1.12.5/src/context/example_test.go
new file mode 100644
index 0000000..2b28b57
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/context/example_test.go
@@ -0,0 +1,118 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package context_test
+
+import (
+	"context"
+	"fmt"
+	"time"
+)
+
+// This example demonstrates the use of a cancelable context to prevent a
+// goroutine leak. By the end of the example function, the goroutine started
+// by gen will return without leaking.
+func ExampleWithCancel() {
+	// gen generates integers in a separate goroutine and
+	// sends them to the returned channel.
+	// The callers of gen need to cancel the context once
+	// they are done consuming generated integers not to leak
+	// the internal goroutine started by gen.
+	gen := func(ctx context.Context) <-chan int {
+		dst := make(chan int)
+		n := 1
+		go func() {
+			for {
+				select {
+				case <-ctx.Done():
+					return // returning not to leak the goroutine
+				case dst <- n:
+					n++
+				}
+			}
+		}()
+		return dst
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel() // cancel when we are finished consuming integers
+
+	for n := range gen(ctx) {
+		fmt.Println(n)
+		if n == 5 {
+			break
+		}
+	}
+	// Output:
+	// 1
+	// 2
+	// 3
+	// 4
+	// 5
+}
+
+// This example passes a context with an arbitrary deadline to tell a blocking
+// function that it should abandon its work as soon as it gets to it.
+func ExampleWithDeadline() {
+	d := time.Now().Add(50 * time.Millisecond)
+	ctx, cancel := context.WithDeadline(context.Background(), d)
+
+	// Even though ctx will be expired, it is good practice to call its
+	// cancelation function in any case. Failure to do so may keep the
+	// context and its parent alive longer than necessary.
+	defer cancel()
+
+	select {
+	case <-time.After(1 * time.Second):
+		fmt.Println("overslept")
+	case <-ctx.Done():
+		fmt.Println(ctx.Err())
+	}
+
+	// Output:
+	// context deadline exceeded
+}
+
+// This example passes a context with a timeout to tell a blocking function that
+// it should abandon its work after the timeout elapses.
+func ExampleWithTimeout() {
+	// Pass a context with a timeout to tell a blocking function that it
+	// should abandon its work after the timeout elapses.
+	ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
+	defer cancel()
+
+	select {
+	case <-time.After(1 * time.Second):
+		fmt.Println("overslept")
+	case <-ctx.Done():
+		fmt.Println(ctx.Err()) // prints "context deadline exceeded"
+	}
+
+	// Output:
+	// context deadline exceeded
+}
+
+// This example demonstrates how a value can be passed to the context
+// and also how to retrieve it if it exists.
+func ExampleWithValue() {
+	type favContextKey string
+
+	f := func(ctx context.Context, k favContextKey) {
+		if v := ctx.Value(k); v != nil {
+			fmt.Println("found value:", v)
+			return
+		}
+		fmt.Println("key not found:", k)
+	}
+
+	k := favContextKey("language")
+	ctx := context.WithValue(context.Background(), k, "Go")
+
+	f(ctx, k)
+	f(ctx, favContextKey("color"))
+
+	// Output:
+	// found value: Go
+	// key not found: color
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/context/net_test.go b/internal/stdlib/testdata/v1.12.5/src/context/net_test.go
new file mode 100644
index 0000000..a007689
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/context/net_test.go
@@ -0,0 +1,21 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package context_test
+
+import (
+	"context"
+	"net"
+	"testing"
+)
+
+func TestDeadlineExceededIsNetError(t *testing.T) {
+	err, ok := context.DeadlineExceeded.(net.Error)
+	if !ok {
+		t.Fatal("DeadlineExceeded does not implement net.Error")
+	}
+	if !err.Timeout() || !err.Temporary() {
+		t.Fatalf("Timeout() = %v, Temporary() = %v, want true, true", err.Timeout(), err.Temporary())
+	}
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/context/x_test.go b/internal/stdlib/testdata/v1.12.5/src/context/x_test.go
new file mode 100644
index 0000000..d14b6f1
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/context/x_test.go
@@ -0,0 +1,29 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package context_test
+
+import (
+	. "context"
+	"testing"
+)
+
+func TestBackground(t *testing.T)                      { XTestBackground(t) }
+func TestTODO(t *testing.T)                            { XTestTODO(t) }
+func TestWithCancel(t *testing.T)                      { XTestWithCancel(t) }
+func TestParentFinishesChild(t *testing.T)             { XTestParentFinishesChild(t) }
+func TestChildFinishesFirst(t *testing.T)              { XTestChildFinishesFirst(t) }
+func TestDeadline(t *testing.T)                        { XTestDeadline(t) }
+func TestTimeout(t *testing.T)                         { XTestTimeout(t) }
+func TestCanceledTimeout(t *testing.T)                 { XTestCanceledTimeout(t) }
+func TestValues(t *testing.T)                          { XTestValues(t) }
+func TestAllocs(t *testing.T)                          { XTestAllocs(t, testing.Short, testing.AllocsPerRun) }
+func TestSimultaneousCancels(t *testing.T)             { XTestSimultaneousCancels(t) }
+func TestInterlockedCancels(t *testing.T)              { XTestInterlockedCancels(t) }
+func TestLayersCancel(t *testing.T)                    { XTestLayersCancel(t) }
+func TestLayersTimeout(t *testing.T)                   { XTestLayersTimeout(t) }
+func TestCancelRemoves(t *testing.T)                   { XTestCancelRemoves(t) }
+func TestWithCancelCanceledParent(t *testing.T)        { XTestWithCancelCanceledParent(t) }
+func TestWithValueChecksKey(t *testing.T)              { XTestWithValueChecksKey(t) }
+func TestDeadlineExceededSupportsTimeout(t *testing.T) { XTestDeadlineExceededSupportsTimeout(t) }
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/bench_test.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/bench_test.go
new file mode 100644
index 0000000..6a72e77
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/bench_test.go
@@ -0,0 +1,363 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Large data benchmark.
+// The JSON data is a summary of agl's changes in the
+// go, webkit, and chromium open source projects.
+// We benchmark converting between the JSON form
+// and in-memory data structures.
+
+package json
+
+import (
+	"bytes"
+	"compress/gzip"
+	"fmt"
+	"internal/testenv"
+	"io"
+	"os"
+	"reflect"
+	"runtime"
+	"strings"
+	"sync"
+	"testing"
+)
+
+type codeResponse struct {
+	Tree     *codeNode `json:"tree"`
+	Username string    `json:"username"`
+}
+
+type codeNode struct {
+	Name     string      `json:"name"`
+	Kids     []*codeNode `json:"kids"`
+	CLWeight float64     `json:"cl_weight"`
+	Touches  int         `json:"touches"`
+	MinT     int64       `json:"min_t"`
+	MaxT     int64       `json:"max_t"`
+	MeanT    int64       `json:"mean_t"`
+}
+
+var codeJSON []byte
+var codeStruct codeResponse
+
+func codeInit() {
+	f, err := os.Open("testdata/code.json.gz")
+	if err != nil {
+		panic(err)
+	}
+	defer f.Close()
+	gz, err := gzip.NewReader(f)
+	if err != nil {
+		panic(err)
+	}
+	data, err := io.ReadAll(gz)
+	if err != nil {
+		panic(err)
+	}
+
+	codeJSON = data
+
+	if err := Unmarshal(codeJSON, &codeStruct); err != nil {
+		panic("unmarshal code.json: " + err.Error())
+	}
+
+	if data, err = Marshal(&codeStruct); err != nil {
+		panic("marshal code.json: " + err.Error())
+	}
+
+	if !bytes.Equal(data, codeJSON) {
+		println("different lengths", len(data), len(codeJSON))
+		for i := 0; i < len(data) && i < len(codeJSON); i++ {
+			if data[i] != codeJSON[i] {
+				println("re-marshal: changed at byte", i)
+				println("orig: ", string(codeJSON[i-10:i+10]))
+				println("new: ", string(data[i-10:i+10]))
+				break
+			}
+		}
+		panic("re-marshal code.json: different result")
+	}
+}
+
+func BenchmarkCodeEncoder(b *testing.B) {
+	if codeJSON == nil {
+		b.StopTimer()
+		codeInit()
+		b.StartTimer()
+	}
+	b.RunParallel(func(pb *testing.PB) {
+		enc := NewEncoder(io.Discard)
+		for pb.Next() {
+			if err := enc.Encode(&codeStruct); err != nil {
+				b.Fatal("Encode:", err)
+			}
+		}
+	})
+	b.SetBytes(int64(len(codeJSON)))
+}
+
+func BenchmarkCodeMarshal(b *testing.B) {
+	if codeJSON == nil {
+		b.StopTimer()
+		codeInit()
+		b.StartTimer()
+	}
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			if _, err := Marshal(&codeStruct); err != nil {
+				b.Fatal("Marshal:", err)
+			}
+		}
+	})
+	b.SetBytes(int64(len(codeJSON)))
+}
+
+func benchMarshalBytes(n int) func(*testing.B) {
+	sample := []byte("hello world")
+	// Use a struct pointer, to avoid an allocation when passing it as an
+	// interface parameter to Marshal.
+	v := &struct {
+		Bytes []byte
+	}{
+		bytes.Repeat(sample, (n/len(sample))+1)[:n],
+	}
+	return func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			if _, err := Marshal(v); err != nil {
+				b.Fatal("Marshal:", err)
+			}
+		}
+	}
+}
+
+func BenchmarkMarshalBytes(b *testing.B) {
+	// 32 fits within encodeState.scratch.
+	b.Run("32", benchMarshalBytes(32))
+	// 256 doesn't fit in encodeState.scratch, but is small enough to
+	// allocate and avoid the slower base64.NewEncoder.
+	b.Run("256", benchMarshalBytes(256))
+	// 4096 is large enough that we want to avoid allocating for it.
+	b.Run("4096", benchMarshalBytes(4096))
+}
+
+func BenchmarkCodeDecoder(b *testing.B) {
+	if codeJSON == nil {
+		b.StopTimer()
+		codeInit()
+		b.StartTimer()
+	}
+	b.RunParallel(func(pb *testing.PB) {
+		var buf bytes.Buffer
+		dec := NewDecoder(&buf)
+		var r codeResponse
+		for pb.Next() {
+			buf.Write(codeJSON)
+			// hide EOF
+			buf.WriteByte('\n')
+			buf.WriteByte('\n')
+			buf.WriteByte('\n')
+			if err := dec.Decode(&r); err != nil {
+				b.Fatal("Decode:", err)
+			}
+		}
+	})
+	b.SetBytes(int64(len(codeJSON)))
+}
+
+func BenchmarkUnicodeDecoder(b *testing.B) {
+	j := []byte(`"\uD83D\uDE01"`)
+	b.SetBytes(int64(len(j)))
+	r := bytes.NewReader(j)
+	dec := NewDecoder(r)
+	var out string
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		if err := dec.Decode(&out); err != nil {
+			b.Fatal("Decode:", err)
+		}
+		r.Seek(0, 0)
+	}
+}
+
+func BenchmarkDecoderStream(b *testing.B) {
+	b.StopTimer()
+	var buf bytes.Buffer
+	dec := NewDecoder(&buf)
+	buf.WriteString(`"` + strings.Repeat("x", 1000000) + `"` + "\n\n\n")
+	var x interface{}
+	if err := dec.Decode(&x); err != nil {
+		b.Fatal("Decode:", err)
+	}
+	ones := strings.Repeat(" 1\n", 300000) + "\n\n\n"
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		if i%300000 == 0 {
+			buf.WriteString(ones)
+		}
+		x = nil
+		if err := dec.Decode(&x); err != nil || x != 1.0 {
+			b.Fatalf("Decode: %v after %d", err, i)
+		}
+	}
+}
+
+func BenchmarkCodeUnmarshal(b *testing.B) {
+	if codeJSON == nil {
+		b.StopTimer()
+		codeInit()
+		b.StartTimer()
+	}
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			var r codeResponse
+			if err := Unmarshal(codeJSON, &r); err != nil {
+				b.Fatal("Unmarshal:", err)
+			}
+		}
+	})
+	b.SetBytes(int64(len(codeJSON)))
+}
+
+func BenchmarkCodeUnmarshalReuse(b *testing.B) {
+	if codeJSON == nil {
+		b.StopTimer()
+		codeInit()
+		b.StartTimer()
+	}
+	b.RunParallel(func(pb *testing.PB) {
+		var r codeResponse
+		for pb.Next() {
+			if err := Unmarshal(codeJSON, &r); err != nil {
+				b.Fatal("Unmarshal:", err)
+			}
+		}
+	})
+	// TODO(bcmills): Is there a missing b.SetBytes here?
+}
+
+func BenchmarkUnmarshalString(b *testing.B) {
+	data := []byte(`"hello, world"`)
+	b.RunParallel(func(pb *testing.PB) {
+		var s string
+		for pb.Next() {
+			if err := Unmarshal(data, &s); err != nil {
+				b.Fatal("Unmarshal:", err)
+			}
+		}
+	})
+}
+
+func BenchmarkUnmarshalFloat64(b *testing.B) {
+	data := []byte(`3.14`)
+	b.RunParallel(func(pb *testing.PB) {
+		var f float64
+		for pb.Next() {
+			if err := Unmarshal(data, &f); err != nil {
+				b.Fatal("Unmarshal:", err)
+			}
+		}
+	})
+}
+
+func BenchmarkUnmarshalInt64(b *testing.B) {
+	data := []byte(`3`)
+	b.RunParallel(func(pb *testing.PB) {
+		var x int64
+		for pb.Next() {
+			if err := Unmarshal(data, &x); err != nil {
+				b.Fatal("Unmarshal:", err)
+			}
+		}
+	})
+}
+
+func BenchmarkIssue10335(b *testing.B) {
+	b.ReportAllocs()
+	j := []byte(`{"a":{ }}`)
+	b.RunParallel(func(pb *testing.PB) {
+		var s struct{}
+		for pb.Next() {
+			if err := Unmarshal(j, &s); err != nil {
+				b.Fatal(err)
+			}
+		}
+	})
+}
+
+func BenchmarkUnmapped(b *testing.B) {
+	b.ReportAllocs()
+	j := []byte(`{"s": "hello", "y": 2, "o": {"x": 0}, "a": [1, 99, {"x": 1}]}`)
+	b.RunParallel(func(pb *testing.PB) {
+		var s struct{}
+		for pb.Next() {
+			if err := Unmarshal(j, &s); err != nil {
+				b.Fatal(err)
+			}
+		}
+	})
+}
+
+func BenchmarkTypeFieldsCache(b *testing.B) {
+	var maxTypes int = 1e6
+	if testenv.Builder() != "" {
+		maxTypes = 1e3 // restrict cache sizes on builders
+	}
+
+	// Dynamically generate many new types.
+	types := make([]reflect.Type, maxTypes)
+	fs := []reflect.StructField{{
+		Type:  reflect.TypeOf(""),
+		Index: []int{0},
+	}}
+	for i := range types {
+		fs[0].Name = fmt.Sprintf("TypeFieldsCache%d", i)
+		types[i] = reflect.StructOf(fs)
+	}
+
+	// clearClear clears the cache. Other JSON operations, must not be running.
+	clearCache := func() {
+		fieldCache = sync.Map{}
+	}
+
+	// MissTypes tests the performance of repeated cache misses.
+	// This measures the time to rebuild a cache of size nt.
+	for nt := 1; nt <= maxTypes; nt *= 10 {
+		ts := types[:nt]
+		b.Run(fmt.Sprintf("MissTypes%d", nt), func(b *testing.B) {
+			nc := runtime.GOMAXPROCS(0)
+			for i := 0; i < b.N; i++ {
+				clearCache()
+				var wg sync.WaitGroup
+				for j := 0; j < nc; j++ {
+					wg.Add(1)
+					go func(j int) {
+						for _, t := range ts[(j*len(ts))/nc : ((j+1)*len(ts))/nc] {
+							cachedTypeFields(t)
+						}
+						wg.Done()
+					}(j)
+				}
+				wg.Wait()
+			}
+		})
+	}
+
+	// HitTypes tests the performance of repeated cache hits.
+	// This measures the average time of each cache lookup.
+	for nt := 1; nt <= maxTypes; nt *= 10 {
+		// Pre-warm a cache of size nt.
+		clearCache()
+		for _, t := range types[:nt] {
+			cachedTypeFields(t)
+		}
+		b.Run(fmt.Sprintf("HitTypes%d", nt), func(b *testing.B) {
+			b.RunParallel(func(pb *testing.PB) {
+				for pb.Next() {
+					cachedTypeFields(types[0])
+				}
+			})
+		})
+	}
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/decode.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/decode.go
new file mode 100644
index 0000000..731553d
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/decode.go
@@ -0,0 +1,1292 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Represents JSON data structure using native Go types: booleans, floats,
+// strings, arrays, and maps.
+
+package json
+
+import (
+	"bytes"
+	"encoding"
+	"encoding/base64"
+	"fmt"
+	"reflect"
+	"strconv"
+	"unicode"
+	"unicode/utf16"
+	"unicode/utf8"
+)
+
+// Unmarshal parses the JSON-encoded data and stores the result
+// in the value pointed to by v. If v is nil or not a pointer,
+// Unmarshal returns an InvalidUnmarshalError.
+//
+// Unmarshal uses the inverse of the encodings that
+// Marshal uses, allocating maps, slices, and pointers as necessary,
+// with the following additional rules:
+//
+// To unmarshal JSON into a pointer, Unmarshal first handles the case of
+// the JSON being the JSON literal null. In that case, Unmarshal sets
+// the pointer to nil. Otherwise, Unmarshal unmarshals the JSON into
+// the value pointed at by the pointer. If the pointer is nil, Unmarshal
+// allocates a new value for it to point to.
+//
+// To unmarshal JSON into a value implementing the Unmarshaler interface,
+// Unmarshal calls that value's UnmarshalJSON method, including
+// when the input is a JSON null.
+// Otherwise, if the value implements encoding.TextUnmarshaler
+// and the input is a JSON quoted string, Unmarshal calls that value's
+// UnmarshalText method with the unquoted form of the string.
+//
+// To unmarshal JSON into a struct, Unmarshal matches incoming object
+// keys to the keys used by Marshal (either the struct field name or its tag),
+// preferring an exact match but also accepting a case-insensitive match. By
+// default, object keys which don't have a corresponding struct field are
+// ignored (see Decoder.DisallowUnknownFields for an alternative).
+//
+// To unmarshal JSON into an interface value,
+// Unmarshal stores one of these in the interface value:
+//
+//	bool, for JSON booleans
+//	float64, for JSON numbers
+//	string, for JSON strings
+//	[]interface{}, for JSON arrays
+//	map[string]interface{}, for JSON objects
+//	nil for JSON null
+//
+// To unmarshal a JSON array into a slice, Unmarshal resets the slice length
+// to zero and then appends each element to the slice.
+// As a special case, to unmarshal an empty JSON array into a slice,
+// Unmarshal replaces the slice with a new empty slice.
+//
+// To unmarshal a JSON array into a Go array, Unmarshal decodes
+// JSON array elements into corresponding Go array elements.
+// If the Go array is smaller than the JSON array,
+// the additional JSON array elements are discarded.
+// If the JSON array is smaller than the Go array,
+// the additional Go array elements are set to zero values.
+//
+// To unmarshal a JSON object into a map, Unmarshal first establishes a map to
+// use. If the map is nil, Unmarshal allocates a new map. Otherwise Unmarshal
+// reuses the existing map, keeping existing entries. Unmarshal then stores
+// key-value pairs from the JSON object into the map. The map's key type must
+// either be a string, an integer, or implement encoding.TextUnmarshaler.
+//
+// If a JSON value is not appropriate for a given target type,
+// or if a JSON number overflows the target type, Unmarshal
+// skips that field and completes the unmarshaling as best it can.
+// If no more serious errors are encountered, Unmarshal returns
+// an UnmarshalTypeError describing the earliest such error. In any
+// case, it's not guaranteed that all the remaining fields following
+// the problematic one will be unmarshaled into the target object.
+//
+// The JSON null value unmarshals into an interface, map, pointer, or slice
+// by setting that Go value to nil. Because null is often used in JSON to mean
+// ``not present,'' unmarshaling a JSON null into any other Go type has no effect
+// on the value and produces no error.
+//
+// When unmarshaling quoted strings, invalid UTF-8 or
+// invalid UTF-16 surrogate pairs are not treated as an error.
+// Instead, they are replaced by the Unicode replacement
+// character U+FFFD.
+//
+func Unmarshal(data []byte, v interface{}) error {
+	// Check for well-formedness.
+	// Avoids filling out half a data structure
+	// before discovering a JSON syntax error.
+	var d decodeState
+	err := checkValid(data, &d.scan)
+	if err != nil {
+		return err
+	}
+
+	d.init(data)
+	return d.unmarshal(v)
+}
+
+// Unmarshaler is the interface implemented by types
+// that can unmarshal a JSON description of themselves.
+// The input can be assumed to be a valid encoding of
+// a JSON value. UnmarshalJSON must copy the JSON data
+// if it wishes to retain the data after returning.
+//
+// By convention, to approximate the behavior of Unmarshal itself,
+// Unmarshalers implement UnmarshalJSON([]byte("null")) as a no-op.
+type Unmarshaler interface {
+	UnmarshalJSON([]byte) error
+}
+
+// An UnmarshalTypeError describes a JSON value that was
+// not appropriate for a value of a specific Go type.
+type UnmarshalTypeError struct {
+	Value  string       // description of JSON value - "bool", "array", "number -5"
+	Type   reflect.Type // type of Go value it could not be assigned to
+	Offset int64        // error occurred after reading Offset bytes
+	Struct string       // name of the struct type containing the field
+	Field  string       // name of the field holding the Go value
+}
+
+func (e *UnmarshalTypeError) Error() string {
+	if e.Struct != "" || e.Field != "" {
+		return "json: cannot unmarshal " + e.Value + " into Go struct field " + e.Struct + "." + e.Field + " of type " + e.Type.String()
+	}
+	return "json: cannot unmarshal " + e.Value + " into Go value of type " + e.Type.String()
+}
+
+// An UnmarshalFieldError describes a JSON object key that
+// led to an unexported (and therefore unwritable) struct field.
+//
+// Deprecated: No longer used; kept for compatibility.
+type UnmarshalFieldError struct {
+	Key   string
+	Type  reflect.Type
+	Field reflect.StructField
+}
+
+func (e *UnmarshalFieldError) Error() string {
+	return "json: cannot unmarshal object key " + strconv.Quote(e.Key) + " into unexported field " + e.Field.Name + " of type " + e.Type.String()
+}
+
+// An InvalidUnmarshalError describes an invalid argument passed to Unmarshal.
+// (The argument to Unmarshal must be a non-nil pointer.)
+type InvalidUnmarshalError struct {
+	Type reflect.Type
+}
+
+func (e *InvalidUnmarshalError) Error() string {
+	if e.Type == nil {
+		return "json: Unmarshal(nil)"
+	}
+
+	if e.Type.Kind() != reflect.Ptr {
+		return "json: Unmarshal(non-pointer " + e.Type.String() + ")"
+	}
+	return "json: Unmarshal(nil " + e.Type.String() + ")"
+}
+
+func (d *decodeState) unmarshal(v interface{}) error {
+	rv := reflect.ValueOf(v)
+	if rv.Kind() != reflect.Ptr || rv.IsNil() {
+		return &InvalidUnmarshalError{reflect.TypeOf(v)}
+	}
+
+	d.scan.reset()
+	d.scanWhile(scanSkipSpace)
+	// We decode rv not rv.Elem because the Unmarshaler interface
+	// test must be applied at the top level of the value.
+	err := d.value(rv)
+	if err != nil {
+		return d.addErrorContext(err)
+	}
+	return d.savedError
+}
+
+// A Number represents a JSON number literal.
+type Number string
+
+// String returns the literal text of the number.
+func (n Number) String() string { return string(n) }
+
+// Float64 returns the number as a float64.
+func (n Number) Float64() (float64, error) {
+	return strconv.ParseFloat(string(n), 64)
+}
+
+// Int64 returns the number as an int64.
+func (n Number) Int64() (int64, error) {
+	return strconv.ParseInt(string(n), 10, 64)
+}
+
+// isValidNumber reports whether s is a valid JSON number literal.
+func isValidNumber(s string) bool {
+	// This function implements the JSON numbers grammar.
+	// See https://tools.ietf.org/html/rfc7159#section-6
+	// and https://json.org/number.gif
+
+	if s == "" {
+		return false
+	}
+
+	// Optional -
+	if s[0] == '-' {
+		s = s[1:]
+		if s == "" {
+			return false
+		}
+	}
+
+	// Digits
+	switch {
+	default:
+		return false
+
+	case s[0] == '0':
+		s = s[1:]
+
+	case '1' <= s[0] && s[0] <= '9':
+		s = s[1:]
+		for len(s) > 0 && '0' <= s[0] && s[0] <= '9' {
+			s = s[1:]
+		}
+	}
+
+	// . followed by 1 or more digits.
+	if len(s) >= 2 && s[0] == '.' && '0' <= s[1] && s[1] <= '9' {
+		s = s[2:]
+		for len(s) > 0 && '0' <= s[0] && s[0] <= '9' {
+			s = s[1:]
+		}
+	}
+
+	// e or E followed by an optional - or + and
+	// 1 or more digits.
+	if len(s) >= 2 && (s[0] == 'e' || s[0] == 'E') {
+		s = s[1:]
+		if s[0] == '+' || s[0] == '-' {
+			s = s[1:]
+			if s == "" {
+				return false
+			}
+		}
+		for len(s) > 0 && '0' <= s[0] && s[0] <= '9' {
+			s = s[1:]
+		}
+	}
+
+	// Make sure we are at the end.
+	return s == ""
+}
+
+// decodeState represents the state while decoding a JSON value.
+type decodeState struct {
+	data         []byte
+	off          int // next read offset in data
+	opcode       int // last read result
+	scan         scanner
+	errorContext struct { // provides context for type errors
+		Struct reflect.Type
+		Field  string
+	}
+	savedError            error
+	useNumber             bool
+	disallowUnknownFields bool
+}
+
+// readIndex returns the position of the last byte read.
+func (d *decodeState) readIndex() int {
+	return d.off - 1
+}
+
+// phasePanicMsg is used as a panic message when we end up with something that
+// shouldn't happen. It can indicate a bug in the JSON decoder, or that
+// something is editing the data slice while the decoder executes.
+const phasePanicMsg = "JSON decoder out of sync - data changing underfoot?"
+
+func (d *decodeState) init(data []byte) *decodeState {
+	d.data = data
+	d.off = 0
+	d.savedError = nil
+	d.errorContext.Struct = nil
+	d.errorContext.Field = ""
+	return d
+}
+
+// saveError saves the first err it is called with,
+// for reporting at the end of the unmarshal.
+func (d *decodeState) saveError(err error) {
+	if d.savedError == nil {
+		d.savedError = d.addErrorContext(err)
+	}
+}
+
+// addErrorContext returns a new error enhanced with information from d.errorContext
+func (d *decodeState) addErrorContext(err error) error {
+	if d.errorContext.Struct != nil || d.errorContext.Field != "" {
+		switch err := err.(type) {
+		case *UnmarshalTypeError:
+			err.Struct = d.errorContext.Struct.Name()
+			err.Field = d.errorContext.Field
+			return err
+		}
+	}
+	return err
+}
+
+// skip scans to the end of what was started.
+func (d *decodeState) skip() {
+	s, data, i := &d.scan, d.data, d.off
+	depth := len(s.parseState)
+	for {
+		op := s.step(s, data[i])
+		i++
+		if len(s.parseState) < depth {
+			d.off = i
+			d.opcode = op
+			return
+		}
+	}
+}
+
+// scanNext processes the byte at d.data[d.off].
+func (d *decodeState) scanNext() {
+	if d.off < len(d.data) {
+		d.opcode = d.scan.step(&d.scan, d.data[d.off])
+		d.off++
+	} else {
+		d.opcode = d.scan.eof()
+		d.off = len(d.data) + 1 // mark processed EOF with len+1
+	}
+}
+
+// scanWhile processes bytes in d.data[d.off:] until it
+// receives a scan code not equal to op.
+func (d *decodeState) scanWhile(op int) {
+	s, data, i := &d.scan, d.data, d.off
+	for i < len(data) {
+		newOp := s.step(s, data[i])
+		i++
+		if newOp != op {
+			d.opcode = newOp
+			d.off = i
+			return
+		}
+	}
+
+	d.off = len(data) + 1 // mark processed EOF with len+1
+	d.opcode = d.scan.eof()
+}
+
+// value consumes a JSON value from d.data[d.off-1:], decoding into v, and
+// reads the following byte ahead. If v is invalid, the value is discarded.
+// The first byte of the value has been read already.
+func (d *decodeState) value(v reflect.Value) error {
+	switch d.opcode {
+	default:
+		panic(phasePanicMsg)
+
+	case scanBeginArray:
+		if v.IsValid() {
+			if err := d.array(v); err != nil {
+				return err
+			}
+		} else {
+			d.skip()
+		}
+		d.scanNext()
+
+	case scanBeginObject:
+		if v.IsValid() {
+			if err := d.object(v); err != nil {
+				return err
+			}
+		} else {
+			d.skip()
+		}
+		d.scanNext()
+
+	case scanBeginLiteral:
+		// All bytes inside literal return scanContinue op code.
+		start := d.readIndex()
+		d.scanWhile(scanContinue)
+
+		if v.IsValid() {
+			if err := d.literalStore(d.data[start:d.readIndex()], v, false); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+type unquotedValue struct{}
+
+// valueQuoted is like value but decodes a
+// quoted string literal or literal null into an interface value.
+// If it finds anything other than a quoted string literal or null,
+// valueQuoted returns unquotedValue{}.
+func (d *decodeState) valueQuoted() interface{} {
+	switch d.opcode {
+	default:
+		panic(phasePanicMsg)
+
+	case scanBeginArray, scanBeginObject:
+		d.skip()
+		d.scanNext()
+
+	case scanBeginLiteral:
+		v := d.literalInterface()
+		switch v.(type) {
+		case nil, string:
+			return v
+		}
+	}
+	return unquotedValue{}
+}
+
+// indirect walks down v allocating pointers as needed,
+// until it gets to a non-pointer.
+// if it encounters an Unmarshaler, indirect stops and returns that.
+// if decodingNull is true, indirect stops at the last pointer so it can be set to nil.
+func indirect(v reflect.Value, decodingNull bool) (Unmarshaler, encoding.TextUnmarshaler, reflect.Value) {
+	// Issue #24153 indicates that it is generally not a guaranteed property
+	// that you may round-trip a reflect.Value by calling Value.Addr().Elem()
+	// and expect the value to still be settable for values derived from
+	// unexported embedded struct fields.
+	//
+	// The logic below effectively does this when it first addresses the value
+	// (to satisfy possible pointer methods) and continues to dereference
+	// subsequent pointers as necessary.
+	//
+	// After the first round-trip, we set v back to the original value to
+	// preserve the original RW flags contained in reflect.Value.
+	v0 := v
+	haveAddr := false
+
+	// If v is a named type and is addressable,
+	// start with its address, so that if the type has pointer methods,
+	// we find them.
+	if v.Kind() != reflect.Ptr && v.Type().Name() != "" && v.CanAddr() {
+		haveAddr = true
+		v = v.Addr()
+	}
+	for {
+		// Load value from interface, but only if the result will be
+		// usefully addressable.
+		if v.Kind() == reflect.Interface && !v.IsNil() {
+			e := v.Elem()
+			if e.Kind() == reflect.Ptr && !e.IsNil() && (!decodingNull || e.Elem().Kind() == reflect.Ptr) {
+				haveAddr = false
+				v = e
+				continue
+			}
+		}
+
+		if v.Kind() != reflect.Ptr {
+			break
+		}
+
+		if v.Elem().Kind() != reflect.Ptr && decodingNull && v.CanSet() {
+			break
+		}
+		if v.IsNil() {
+			v.Set(reflect.New(v.Type().Elem()))
+		}
+		if v.Type().NumMethod() > 0 && v.CanInterface() {
+			if u, ok := v.Interface().(Unmarshaler); ok {
+				return u, nil, reflect.Value{}
+			}
+			if !decodingNull {
+				if u, ok := v.Interface().(encoding.TextUnmarshaler); ok {
+					return nil, u, reflect.Value{}
+				}
+			}
+		}
+
+		if haveAddr {
+			v = v0 // restore original value after round-trip Value.Addr().Elem()
+			haveAddr = false
+		} else {
+			v = v.Elem()
+		}
+	}
+	return nil, nil, v
+}
+
+// array consumes an array from d.data[d.off-1:], decoding into v.
+// The first byte of the array ('[') has been read already.
+func (d *decodeState) array(v reflect.Value) error {
+	// Check for unmarshaler.
+	u, ut, pv := indirect(v, false)
+	if u != nil {
+		start := d.readIndex()
+		d.skip()
+		return u.UnmarshalJSON(d.data[start:d.off])
+	}
+	if ut != nil {
+		d.saveError(&UnmarshalTypeError{Value: "array", Type: v.Type(), Offset: int64(d.off)})
+		d.skip()
+		return nil
+	}
+	v = pv
+
+	// Check type of target.
+	switch v.Kind() {
+	case reflect.Interface:
+		if v.NumMethod() == 0 {
+			// Decoding into nil interface? Switch to non-reflect code.
+			ai := d.arrayInterface()
+			v.Set(reflect.ValueOf(ai))
+			return nil
+		}
+		// Otherwise it's invalid.
+		fallthrough
+	default:
+		d.saveError(&UnmarshalTypeError{Value: "array", Type: v.Type(), Offset: int64(d.off)})
+		d.skip()
+		return nil
+	case reflect.Array, reflect.Slice:
+		break
+	}
+
+	i := 0
+	for {
+		// Look ahead for ] - can only happen on first iteration.
+		d.scanWhile(scanSkipSpace)
+		if d.opcode == scanEndArray {
+			break
+		}
+
+		// Get element of array, growing if necessary.
+		if v.Kind() == reflect.Slice {
+			// Grow slice if necessary
+			if i >= v.Cap() {
+				newcap := v.Cap() + v.Cap()/2
+				if newcap < 4 {
+					newcap = 4
+				}
+				newv := reflect.MakeSlice(v.Type(), v.Len(), newcap)
+				reflect.Copy(newv, v)
+				v.Set(newv)
+			}
+			if i >= v.Len() {
+				v.SetLen(i + 1)
+			}
+		}
+
+		if i < v.Len() {
+			// Decode into element.
+			if err := d.value(v.Index(i)); err != nil {
+				return err
+			}
+		} else {
+			// Ran out of fixed array: skip.
+			if err := d.value(reflect.Value{}); err != nil {
+				return err
+			}
+		}
+		i++
+
+		// Next token must be , or ].
+		if d.opcode == scanSkipSpace {
+			d.scanWhile(scanSkipSpace)
+		}
+		if d.opcode == scanEndArray {
+			break
+		}
+		if d.opcode != scanArrayValue {
+			panic(phasePanicMsg)
+		}
+	}
+
+	if i < v.Len() {
+		if v.Kind() == reflect.Array {
+			// Array. Zero the rest.
+			z := reflect.Zero(v.Type().Elem())
+			for ; i < v.Len(); i++ {
+				v.Index(i).Set(z)
+			}
+		} else {
+			v.SetLen(i)
+		}
+	}
+	if i == 0 && v.Kind() == reflect.Slice {
+		v.Set(reflect.MakeSlice(v.Type(), 0, 0))
+	}
+	return nil
+}
+
+var nullLiteral = []byte("null")
+var textUnmarshalerType = reflect.TypeOf((*encoding.TextUnmarshaler)(nil)).Elem()
+
+// object consumes an object from d.data[d.off-1:], decoding into v.
+// The first byte ('{') of the object has been read already.
+func (d *decodeState) object(v reflect.Value) error {
+	// Check for unmarshaler.
+	u, ut, pv := indirect(v, false)
+	if u != nil {
+		start := d.readIndex()
+		d.skip()
+		return u.UnmarshalJSON(d.data[start:d.off])
+	}
+	if ut != nil {
+		d.saveError(&UnmarshalTypeError{Value: "object", Type: v.Type(), Offset: int64(d.off)})
+		d.skip()
+		return nil
+	}
+	v = pv
+	t := v.Type()
+
+	// Decoding into nil interface? Switch to non-reflect code.
+	if v.Kind() == reflect.Interface && v.NumMethod() == 0 {
+		oi := d.objectInterface()
+		v.Set(reflect.ValueOf(oi))
+		return nil
+	}
+
+	var fields []field
+
+	// Check type of target:
+	//   struct or
+	//   map[T1]T2 where T1 is string, an integer type,
+	//             or an encoding.TextUnmarshaler
+	switch v.Kind() {
+	case reflect.Map:
+		// Map key must either have string kind, have an integer kind,
+		// or be an encoding.TextUnmarshaler.
+		switch t.Key().Kind() {
+		case reflect.String,
+			reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
+			reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		default:
+			if !reflect.PtrTo(t.Key()).Implements(textUnmarshalerType) {
+				d.saveError(&UnmarshalTypeError{Value: "object", Type: t, Offset: int64(d.off)})
+				d.skip()
+				return nil
+			}
+		}
+		if v.IsNil() {
+			v.Set(reflect.MakeMap(t))
+		}
+	case reflect.Struct:
+		fields = cachedTypeFields(t)
+		// ok
+	default:
+		d.saveError(&UnmarshalTypeError{Value: "object", Type: t, Offset: int64(d.off)})
+		d.skip()
+		return nil
+	}
+
+	var mapElem reflect.Value
+	originalErrorContext := d.errorContext
+
+	for {
+		// Read opening " of string key or closing }.
+		d.scanWhile(scanSkipSpace)
+		if d.opcode == scanEndObject {
+			// closing } - can only happen on first iteration.
+			break
+		}
+		if d.opcode != scanBeginLiteral {
+			panic(phasePanicMsg)
+		}
+
+		// Read key.
+		start := d.readIndex()
+		d.scanWhile(scanContinue)
+		item := d.data[start:d.readIndex()]
+		key, ok := unquoteBytes(item)
+		if !ok {
+			panic(phasePanicMsg)
+		}
+
+		// Figure out field corresponding to key.
+		var subv reflect.Value
+		destring := false // whether the value is wrapped in a string to be decoded first
+
+		if v.Kind() == reflect.Map {
+			elemType := t.Elem()
+			if !mapElem.IsValid() {
+				mapElem = reflect.New(elemType).Elem()
+			} else {
+				mapElem.Set(reflect.Zero(elemType))
+			}
+			subv = mapElem
+		} else {
+			var f *field
+			for i := range fields {
+				ff := &fields[i]
+				if bytes.Equal(ff.nameBytes, key) {
+					f = ff
+					break
+				}
+				if f == nil && ff.equalFold(ff.nameBytes, key) {
+					f = ff
+				}
+			}
+			if f != nil {
+				subv = v
+				destring = f.quoted
+				for _, i := range f.index {
+					if subv.Kind() == reflect.Ptr {
+						if subv.IsNil() {
+							// If a struct embeds a pointer to an unexported type,
+							// it is not possible to set a newly allocated value
+							// since the field is unexported.
+							//
+							// See https://golang.org/issue/21357
+							if !subv.CanSet() {
+								d.saveError(fmt.Errorf("json: cannot set embedded pointer to unexported struct: %v", subv.Type().Elem()))
+								// Invalidate subv to ensure d.value(subv) skips over
+								// the JSON value without assigning it to subv.
+								subv = reflect.Value{}
+								destring = false
+								break
+							}
+							subv.Set(reflect.New(subv.Type().Elem()))
+						}
+						subv = subv.Elem()
+					}
+					subv = subv.Field(i)
+				}
+				d.errorContext.Field = f.name
+				d.errorContext.Struct = t
+			} else if d.disallowUnknownFields {
+				d.saveError(fmt.Errorf("json: unknown field %q", key))
+			}
+		}
+
+		// Read : before value.
+		if d.opcode == scanSkipSpace {
+			d.scanWhile(scanSkipSpace)
+		}
+		if d.opcode != scanObjectKey {
+			panic(phasePanicMsg)
+		}
+		d.scanWhile(scanSkipSpace)
+
+		if destring {
+			switch qv := d.valueQuoted().(type) {
+			case nil:
+				if err := d.literalStore(nullLiteral, subv, false); err != nil {
+					return err
+				}
+			case string:
+				if err := d.literalStore([]byte(qv), subv, true); err != nil {
+					return err
+				}
+			default:
+				d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal unquoted value into %v", subv.Type()))
+			}
+		} else {
+			if err := d.value(subv); err != nil {
+				return err
+			}
+		}
+
+		// Write value back to map;
+		// if using struct, subv points into struct already.
+		if v.Kind() == reflect.Map {
+			kt := t.Key()
+			var kv reflect.Value
+			switch {
+			case kt.Kind() == reflect.String:
+				kv = reflect.ValueOf(key).Convert(kt)
+			case reflect.PtrTo(kt).Implements(textUnmarshalerType):
+				kv = reflect.New(kt)
+				if err := d.literalStore(item, kv, true); err != nil {
+					return err
+				}
+				kv = kv.Elem()
+			default:
+				switch kt.Kind() {
+				case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+					s := string(key)
+					n, err := strconv.ParseInt(s, 10, 64)
+					if err != nil || reflect.Zero(kt).OverflowInt(n) {
+						d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: kt, Offset: int64(start + 1)})
+						break
+					}
+					kv = reflect.ValueOf(n).Convert(kt)
+				case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+					s := string(key)
+					n, err := strconv.ParseUint(s, 10, 64)
+					if err != nil || reflect.Zero(kt).OverflowUint(n) {
+						d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: kt, Offset: int64(start + 1)})
+						break
+					}
+					kv = reflect.ValueOf(n).Convert(kt)
+				default:
+					panic("json: Unexpected key type") // should never occur
+				}
+			}
+			if kv.IsValid() {
+				v.SetMapIndex(kv, subv)
+			}
+		}
+
+		// Next token must be , or }.
+		if d.opcode == scanSkipSpace {
+			d.scanWhile(scanSkipSpace)
+		}
+		if d.opcode == scanEndObject {
+			break
+		}
+		if d.opcode != scanObjectValue {
+			panic(phasePanicMsg)
+		}
+
+		d.errorContext = originalErrorContext
+	}
+	return nil
+}
+
+// convertNumber converts the number literal s to a float64 or a Number
+// depending on the setting of d.useNumber.
+func (d *decodeState) convertNumber(s string) (interface{}, error) {
+	if d.useNumber {
+		return Number(s), nil
+	}
+	f, err := strconv.ParseFloat(s, 64)
+	if err != nil {
+		return nil, &UnmarshalTypeError{Value: "number " + s, Type: reflect.TypeOf(0.0), Offset: int64(d.off)}
+	}
+	return f, nil
+}
+
+var numberType = reflect.TypeOf(Number(""))
+
+// literalStore decodes a literal stored in item into v.
+//
+// fromQuoted indicates whether this literal came from unwrapping a
+// string from the ",string" struct tag option. this is used only to
+// produce more helpful error messages.
+func (d *decodeState) literalStore(item []byte, v reflect.Value, fromQuoted bool) error {
+	// Check for unmarshaler.
+	if len(item) == 0 {
+		//Empty string given
+		d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type()))
+		return nil
+	}
+	isNull := item[0] == 'n' // null
+	u, ut, pv := indirect(v, isNull)
+	if u != nil {
+		return u.UnmarshalJSON(item)
+	}
+	if ut != nil {
+		if item[0] != '"' {
+			if fromQuoted {
+				d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type()))
+				return nil
+			}
+			val := "number"
+			switch item[0] {
+			case 'n':
+				val = "null"
+			case 't', 'f':
+				val = "bool"
+			}
+			d.saveError(&UnmarshalTypeError{Value: val, Type: v.Type(), Offset: int64(d.readIndex())})
+			return nil
+		}
+		s, ok := unquoteBytes(item)
+		if !ok {
+			if fromQuoted {
+				return fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type())
+			}
+			panic(phasePanicMsg)
+		}
+		return ut.UnmarshalText(s)
+	}
+
+	v = pv
+
+	switch c := item[0]; c {
+	case 'n': // null
+		// The main parser checks that only true and false can reach here,
+		// but if this was a quoted string input, it could be anything.
+		if fromQuoted && string(item) != "null" {
+			d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type()))
+			break
+		}
+		switch v.Kind() {
+		case reflect.Interface, reflect.Ptr, reflect.Map, reflect.Slice:
+			v.Set(reflect.Zero(v.Type()))
+			// otherwise, ignore null for primitives/string
+		}
+	case 't', 'f': // true, false
+		value := item[0] == 't'
+		// The main parser checks that only true and false can reach here,
+		// but if this was a quoted string input, it could be anything.
+		if fromQuoted && string(item) != "true" && string(item) != "false" {
+			d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type()))
+			break
+		}
+		switch v.Kind() {
+		default:
+			if fromQuoted {
+				d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type()))
+			} else {
+				d.saveError(&UnmarshalTypeError{Value: "bool", Type: v.Type(), Offset: int64(d.readIndex())})
+			}
+		case reflect.Bool:
+			v.SetBool(value)
+		case reflect.Interface:
+			if v.NumMethod() == 0 {
+				v.Set(reflect.ValueOf(value))
+			} else {
+				d.saveError(&UnmarshalTypeError{Value: "bool", Type: v.Type(), Offset: int64(d.readIndex())})
+			}
+		}
+
+	case '"': // string
+		s, ok := unquoteBytes(item)
+		if !ok {
+			if fromQuoted {
+				return fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type())
+			}
+			panic(phasePanicMsg)
+		}
+		switch v.Kind() {
+		default:
+			d.saveError(&UnmarshalTypeError{Value: "string", Type: v.Type(), Offset: int64(d.readIndex())})
+		case reflect.Slice:
+			if v.Type().Elem().Kind() != reflect.Uint8 {
+				d.saveError(&UnmarshalTypeError{Value: "string", Type: v.Type(), Offset: int64(d.readIndex())})
+				break
+			}
+			b := make([]byte, base64.StdEncoding.DecodedLen(len(s)))
+			n, err := base64.StdEncoding.Decode(b, s)
+			if err != nil {
+				d.saveError(err)
+				break
+			}
+			v.SetBytes(b[:n])
+		case reflect.String:
+			v.SetString(string(s))
+		case reflect.Interface:
+			if v.NumMethod() == 0 {
+				v.Set(reflect.ValueOf(string(s)))
+			} else {
+				d.saveError(&UnmarshalTypeError{Value: "string", Type: v.Type(), Offset: int64(d.readIndex())})
+			}
+		}
+
+	default: // number
+		if c != '-' && (c < '0' || c > '9') {
+			if fromQuoted {
+				return fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type())
+			}
+			panic(phasePanicMsg)
+		}
+		s := string(item)
+		switch v.Kind() {
+		default:
+			if v.Kind() == reflect.String && v.Type() == numberType {
+				v.SetString(s)
+				if !isValidNumber(s) {
+					return fmt.Errorf("json: invalid number literal, trying to unmarshal %q into Number", item)
+				}
+				break
+			}
+			if fromQuoted {
+				return fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type())
+			}
+			d.saveError(&UnmarshalTypeError{Value: "number", Type: v.Type(), Offset: int64(d.readIndex())})
+		case reflect.Interface:
+			n, err := d.convertNumber(s)
+			if err != nil {
+				d.saveError(err)
+				break
+			}
+			if v.NumMethod() != 0 {
+				d.saveError(&UnmarshalTypeError{Value: "number", Type: v.Type(), Offset: int64(d.readIndex())})
+				break
+			}
+			v.Set(reflect.ValueOf(n))
+
+		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+			n, err := strconv.ParseInt(s, 10, 64)
+			if err != nil || v.OverflowInt(n) {
+				d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: v.Type(), Offset: int64(d.readIndex())})
+				break
+			}
+			v.SetInt(n)
+
+		case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+			n, err := strconv.ParseUint(s, 10, 64)
+			if err != nil || v.OverflowUint(n) {
+				d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: v.Type(), Offset: int64(d.readIndex())})
+				break
+			}
+			v.SetUint(n)
+
+		case reflect.Float32, reflect.Float64:
+			n, err := strconv.ParseFloat(s, v.Type().Bits())
+			if err != nil || v.OverflowFloat(n) {
+				d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: v.Type(), Offset: int64(d.readIndex())})
+				break
+			}
+			v.SetFloat(n)
+		}
+	}
+	return nil
+}
+
+// The xxxInterface routines build up a value to be stored
+// in an empty interface. They are not strictly necessary,
+// but they avoid the weight of reflection in this common case.
+
+// valueInterface is like value but returns interface{}
+func (d *decodeState) valueInterface() (val interface{}) {
+	switch d.opcode {
+	default:
+		panic(phasePanicMsg)
+	case scanBeginArray:
+		val = d.arrayInterface()
+		d.scanNext()
+	case scanBeginObject:
+		val = d.objectInterface()
+		d.scanNext()
+	case scanBeginLiteral:
+		val = d.literalInterface()
+	}
+	return
+}
+
+// arrayInterface is like array but returns []interface{}.
+func (d *decodeState) arrayInterface() []interface{} {
+	var v = make([]interface{}, 0)
+	for {
+		// Look ahead for ] - can only happen on first iteration.
+		d.scanWhile(scanSkipSpace)
+		if d.opcode == scanEndArray {
+			break
+		}
+
+		v = append(v, d.valueInterface())
+
+		// Next token must be , or ].
+		if d.opcode == scanSkipSpace {
+			d.scanWhile(scanSkipSpace)
+		}
+		if d.opcode == scanEndArray {
+			break
+		}
+		if d.opcode != scanArrayValue {
+			panic(phasePanicMsg)
+		}
+	}
+	return v
+}
+
+// objectInterface is like object but returns map[string]interface{}.
+func (d *decodeState) objectInterface() map[string]interface{} {
+	m := make(map[string]interface{})
+	for {
+		// Read opening " of string key or closing }.
+		d.scanWhile(scanSkipSpace)
+		if d.opcode == scanEndObject {
+			// closing } - can only happen on first iteration.
+			break
+		}
+		if d.opcode != scanBeginLiteral {
+			panic(phasePanicMsg)
+		}
+
+		// Read string key.
+		start := d.readIndex()
+		d.scanWhile(scanContinue)
+		item := d.data[start:d.readIndex()]
+		key, ok := unquote(item)
+		if !ok {
+			panic(phasePanicMsg)
+		}
+
+		// Read : before value.
+		if d.opcode == scanSkipSpace {
+			d.scanWhile(scanSkipSpace)
+		}
+		if d.opcode != scanObjectKey {
+			panic(phasePanicMsg)
+		}
+		d.scanWhile(scanSkipSpace)
+
+		// Read value.
+		m[key] = d.valueInterface()
+
+		// Next token must be , or }.
+		if d.opcode == scanSkipSpace {
+			d.scanWhile(scanSkipSpace)
+		}
+		if d.opcode == scanEndObject {
+			break
+		}
+		if d.opcode != scanObjectValue {
+			panic(phasePanicMsg)
+		}
+	}
+	return m
+}
+
+// literalInterface consumes and returns a literal from d.data[d.off-1:] and
+// it reads the following byte ahead. The first byte of the literal has been
+// read already (that's how the caller knows it's a literal).
+func (d *decodeState) literalInterface() interface{} {
+	// All bytes inside literal return scanContinue op code.
+	start := d.readIndex()
+	d.scanWhile(scanContinue)
+
+	item := d.data[start:d.readIndex()]
+
+	switch c := item[0]; c {
+	case 'n': // null
+		return nil
+
+	case 't', 'f': // true, false
+		return c == 't'
+
+	case '"': // string
+		s, ok := unquote(item)
+		if !ok {
+			panic(phasePanicMsg)
+		}
+		return s
+
+	default: // number
+		if c != '-' && (c < '0' || c > '9') {
+			panic(phasePanicMsg)
+		}
+		n, err := d.convertNumber(string(item))
+		if err != nil {
+			d.saveError(err)
+		}
+		return n
+	}
+}
+
+// getu4 decodes \uXXXX from the beginning of s, returning the hex value,
+// or it returns -1.
+func getu4(s []byte) rune {
+	if len(s) < 6 || s[0] != '\\' || s[1] != 'u' {
+		return -1
+	}
+	var r rune
+	for _, c := range s[2:6] {
+		switch {
+		case '0' <= c && c <= '9':
+			c = c - '0'
+		case 'a' <= c && c <= 'f':
+			c = c - 'a' + 10
+		case 'A' <= c && c <= 'F':
+			c = c - 'A' + 10
+		default:
+			return -1
+		}
+		r = r*16 + rune(c)
+	}
+	return r
+}
+
+// unquote converts a quoted JSON string literal s into an actual string t.
+// The rules are different than for Go, so cannot use strconv.Unquote.
+func unquote(s []byte) (t string, ok bool) {
+	s, ok = unquoteBytes(s)
+	t = string(s)
+	return
+}
+
+func unquoteBytes(s []byte) (t []byte, ok bool) {
+	if len(s) < 2 || s[0] != '"' || s[len(s)-1] != '"' {
+		return
+	}
+	s = s[1 : len(s)-1]
+
+	// Check for unusual characters. If there are none,
+	// then no unquoting is needed, so return a slice of the
+	// original bytes.
+	r := 0
+	for r < len(s) {
+		c := s[r]
+		if c == '\\' || c == '"' || c < ' ' {
+			break
+		}
+		if c < utf8.RuneSelf {
+			r++
+			continue
+		}
+		rr, size := utf8.DecodeRune(s[r:])
+		if rr == utf8.RuneError && size == 1 {
+			break
+		}
+		r += size
+	}
+	if r == len(s) {
+		return s, true
+	}
+
+	b := make([]byte, len(s)+2*utf8.UTFMax)
+	w := copy(b, s[0:r])
+	for r < len(s) {
+		// Out of room? Can only happen if s is full of
+		// malformed UTF-8 and we're replacing each
+		// byte with RuneError.
+		if w >= len(b)-2*utf8.UTFMax {
+			nb := make([]byte, (len(b)+utf8.UTFMax)*2)
+			copy(nb, b[0:w])
+			b = nb
+		}
+		switch c := s[r]; {
+		case c == '\\':
+			r++
+			if r >= len(s) {
+				return
+			}
+			switch s[r] {
+			default:
+				return
+			case '"', '\\', '/', '\'':
+				b[w] = s[r]
+				r++
+				w++
+			case 'b':
+				b[w] = '\b'
+				r++
+				w++
+			case 'f':
+				b[w] = '\f'
+				r++
+				w++
+			case 'n':
+				b[w] = '\n'
+				r++
+				w++
+			case 'r':
+				b[w] = '\r'
+				r++
+				w++
+			case 't':
+				b[w] = '\t'
+				r++
+				w++
+			case 'u':
+				r--
+				rr := getu4(s[r:])
+				if rr < 0 {
+					return
+				}
+				r += 6
+				if utf16.IsSurrogate(rr) {
+					rr1 := getu4(s[r:])
+					if dec := utf16.DecodeRune(rr, rr1); dec != unicode.ReplacementChar {
+						// A valid pair; consume.
+						r += 6
+						w += utf8.EncodeRune(b[w:], dec)
+						break
+					}
+					// Invalid surrogate; fall back to replacement rune.
+					rr = unicode.ReplacementChar
+				}
+				w += utf8.EncodeRune(b[w:], rr)
+			}
+
+		// Quote, control characters are invalid.
+		case c == '"', c < ' ':
+			return
+
+		// ASCII
+		case c < utf8.RuneSelf:
+			b[w] = c
+			r++
+			w++
+
+		// Coerce to well-formed UTF-8.
+		default:
+			rr, size := utf8.DecodeRune(s[r:])
+			r += size
+			w += utf8.EncodeRune(b[w:], rr)
+		}
+	}
+	return b[0:w], true
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/decode_test.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/decode_test.go
new file mode 100644
index 0000000..5443260
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/decode_test.go
@@ -0,0 +1,2294 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package json
+
+import (
+	"bytes"
+	"encoding"
+	"errors"
+	"fmt"
+	"image"
+	"math"
+	"math/big"
+	"net"
+	"reflect"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+)
+
+type T struct {
+	X string
+	Y int
+	Z int `json:"-"`
+}
+
+type U struct {
+	Alphabet string `json:"alpha"`
+}
+
+type V struct {
+	F1 interface{}
+	F2 int32
+	F3 Number
+	F4 *VOuter
+}
+
+type VOuter struct {
+	V V
+}
+
+type W struct {
+	S SS
+}
+
+type SS string
+
+func (*SS) UnmarshalJSON(data []byte) error {
+	return &UnmarshalTypeError{Value: "number", Type: reflect.TypeOf(SS(""))}
+}
+
+// ifaceNumAsFloat64/ifaceNumAsNumber are used to test unmarshaling with and
+// without UseNumber
+var ifaceNumAsFloat64 = map[string]interface{}{
+	"k1": float64(1),
+	"k2": "s",
+	"k3": []interface{}{float64(1), float64(2.0), float64(3e-3)},
+	"k4": map[string]interface{}{"kk1": "s", "kk2": float64(2)},
+}
+
+var ifaceNumAsNumber = map[string]interface{}{
+	"k1": Number("1"),
+	"k2": "s",
+	"k3": []interface{}{Number("1"), Number("2.0"), Number("3e-3")},
+	"k4": map[string]interface{}{"kk1": "s", "kk2": Number("2")},
+}
+
+type tx struct {
+	x int
+}
+
+type u8 uint8
+
+// A type that can unmarshal itself.
+
+type unmarshaler struct {
+	T bool
+}
+
+func (u *unmarshaler) UnmarshalJSON(b []byte) error {
+	*u = unmarshaler{true} // All we need to see that UnmarshalJSON is called.
+	return nil
+}
+
+type ustruct struct {
+	M unmarshaler
+}
+
+type unmarshalerText struct {
+	A, B string
+}
+
+// needed for re-marshaling tests
+func (u unmarshalerText) MarshalText() ([]byte, error) {
+	return []byte(u.A + ":" + u.B), nil
+}
+
+func (u *unmarshalerText) UnmarshalText(b []byte) error {
+	pos := bytes.IndexByte(b, ':')
+	if pos == -1 {
+		return errors.New("missing separator")
+	}
+	u.A, u.B = string(b[:pos]), string(b[pos+1:])
+	return nil
+}
+
+var _ encoding.TextUnmarshaler = (*unmarshalerText)(nil)
+
+type ustructText struct {
+	M unmarshalerText
+}
+
+// u8marshal is an integer type that can marshal/unmarshal itself.
+type u8marshal uint8
+
+func (u8 u8marshal) MarshalText() ([]byte, error) {
+	return []byte(fmt.Sprintf("u%d", u8)), nil
+}
+
+var errMissingU8Prefix = errors.New("missing 'u' prefix")
+
+func (u8 *u8marshal) UnmarshalText(b []byte) error {
+	if !bytes.HasPrefix(b, []byte{'u'}) {
+		return errMissingU8Prefix
+	}
+	n, err := strconv.Atoi(string(b[1:]))
+	if err != nil {
+		return err
+	}
+	*u8 = u8marshal(n)
+	return nil
+}
+
+var _ encoding.TextUnmarshaler = (*u8marshal)(nil)
+
+var (
+	um0, um1 unmarshaler // target2 of unmarshaling
+	ump      = &um1
+	umtrue   = unmarshaler{true}
+	umslice  = []unmarshaler{{true}}
+	umslicep = new([]unmarshaler)
+	umstruct = ustruct{unmarshaler{true}}
+
+	um0T, um1T   unmarshalerText // target2 of unmarshaling
+	umpType      = &um1T
+	umtrueXY     = unmarshalerText{"x", "y"}
+	umsliceXY    = []unmarshalerText{{"x", "y"}}
+	umslicepType = new([]unmarshalerText)
+	umstructType = new(ustructText)
+	umstructXY   = ustructText{unmarshalerText{"x", "y"}}
+
+	ummapType = map[unmarshalerText]bool{}
+	ummapXY   = map[unmarshalerText]bool{{"x", "y"}: true}
+)
+
+// Test data structures for anonymous fields.
+
+type Point struct {
+	Z int
+}
+
+type Top struct {
+	Level0 int
+	Embed0
+	*Embed0a
+	*Embed0b `json:"e,omitempty"` // treated as named
+	Embed0c  `json:"-"`           // ignored
+	Loop
+	Embed0p // has Point with X, Y, used
+	Embed0q // has Point with Z, used
+	embed   // contains exported field
+}
+
+type Embed0 struct {
+	Level1a int // overridden by Embed0a's Level1a with json tag
+	Level1b int // used because Embed0a's Level1b is renamed
+	Level1c int // used because Embed0a's Level1c is ignored
+	Level1d int // annihilated by Embed0a's Level1d
+	Level1e int `json:"x"` // annihilated by Embed0a.Level1e
+}
+
+type Embed0a struct {
+	Level1a int `json:"Level1a,omitempty"`
+	Level1b int `json:"LEVEL1B,omitempty"`
+	Level1c int `json:"-"`
+	Level1d int // annihilated by Embed0's Level1d
+	Level1f int `json:"x"` // annihilated by Embed0's Level1e
+}
+
+type Embed0b Embed0
+
+type Embed0c Embed0
+
+type Embed0p struct {
+	image.Point
+}
+
+type Embed0q struct {
+	Point
+}
+
+type embed struct {
+	Q int
+}
+
+type Loop struct {
+	Loop1 int `json:",omitempty"`
+	Loop2 int `json:",omitempty"`
+	*Loop
+}
+
+// From reflect test:
+// The X in S6 and S7 annihilate, but they also block the X in S8.S9.
+type S5 struct {
+	S6
+	S7
+	S8
+}
+
+type S6 struct {
+	X int
+}
+
+type S7 S6
+
+type S8 struct {
+	S9
+}
+
+type S9 struct {
+	X int
+	Y int
+}
+
+// From reflect test:
+// The X in S11.S6 and S12.S6 annihilate, but they also block the X in S13.S8.S9.
+type S10 struct {
+	S11
+	S12
+	S13
+}
+
+type S11 struct {
+	S6
+}
+
+type S12 struct {
+	S6
+}
+
+type S13 struct {
+	S8
+}
+
+type Ambig struct {
+	// Given "hello", the first match should win.
+	First  int `json:"HELLO"`
+	Second int `json:"Hello"`
+}
+
+type XYZ struct {
+	X interface{}
+	Y interface{}
+	Z interface{}
+}
+
+type unexportedWithMethods struct{}
+
+func (unexportedWithMethods) F() {}
+
+func sliceAddr(x []int) *[]int                 { return &x }
+func mapAddr(x map[string]int) *map[string]int { return &x }
+
+type byteWithMarshalJSON byte
+
+func (b byteWithMarshalJSON) MarshalJSON() ([]byte, error) {
+	return []byte(fmt.Sprintf(`"Z%.2x"`, byte(b))), nil
+}
+
+func (b *byteWithMarshalJSON) UnmarshalJSON(data []byte) error {
+	if len(data) != 5 || data[0] != '"' || data[1] != 'Z' || data[4] != '"' {
+		return fmt.Errorf("bad quoted string")
+	}
+	i, err := strconv.ParseInt(string(data[2:4]), 16, 8)
+	if err != nil {
+		return fmt.Errorf("bad hex")
+	}
+	*b = byteWithMarshalJSON(i)
+	return nil
+}
+
+type byteWithPtrMarshalJSON byte
+
+func (b *byteWithPtrMarshalJSON) MarshalJSON() ([]byte, error) {
+	return byteWithMarshalJSON(*b).MarshalJSON()
+}
+
+func (b *byteWithPtrMarshalJSON) UnmarshalJSON(data []byte) error {
+	return (*byteWithMarshalJSON)(b).UnmarshalJSON(data)
+}
+
+type byteWithMarshalText byte
+
+func (b byteWithMarshalText) MarshalText() ([]byte, error) {
+	return []byte(fmt.Sprintf(`Z%.2x`, byte(b))), nil
+}
+
+func (b *byteWithMarshalText) UnmarshalText(data []byte) error {
+	if len(data) != 3 || data[0] != 'Z' {
+		return fmt.Errorf("bad quoted string")
+	}
+	i, err := strconv.ParseInt(string(data[1:3]), 16, 8)
+	if err != nil {
+		return fmt.Errorf("bad hex")
+	}
+	*b = byteWithMarshalText(i)
+	return nil
+}
+
+type byteWithPtrMarshalText byte
+
+func (b *byteWithPtrMarshalText) MarshalText() ([]byte, error) {
+	return byteWithMarshalText(*b).MarshalText()
+}
+
+func (b *byteWithPtrMarshalText) UnmarshalText(data []byte) error {
+	return (*byteWithMarshalText)(b).UnmarshalText(data)
+}
+
+type intWithMarshalJSON int
+
+func (b intWithMarshalJSON) MarshalJSON() ([]byte, error) {
+	return []byte(fmt.Sprintf(`"Z%.2x"`, int(b))), nil
+}
+
+func (b *intWithMarshalJSON) UnmarshalJSON(data []byte) error {
+	if len(data) != 5 || data[0] != '"' || data[1] != 'Z' || data[4] != '"' {
+		return fmt.Errorf("bad quoted string")
+	}
+	i, err := strconv.ParseInt(string(data[2:4]), 16, 8)
+	if err != nil {
+		return fmt.Errorf("bad hex")
+	}
+	*b = intWithMarshalJSON(i)
+	return nil
+}
+
+type intWithPtrMarshalJSON int
+
+func (b *intWithPtrMarshalJSON) MarshalJSON() ([]byte, error) {
+	return intWithMarshalJSON(*b).MarshalJSON()
+}
+
+func (b *intWithPtrMarshalJSON) UnmarshalJSON(data []byte) error {
+	return (*intWithMarshalJSON)(b).UnmarshalJSON(data)
+}
+
+type intWithMarshalText int
+
+func (b intWithMarshalText) MarshalText() ([]byte, error) {
+	return []byte(fmt.Sprintf(`Z%.2x`, int(b))), nil
+}
+
+func (b *intWithMarshalText) UnmarshalText(data []byte) error {
+	if len(data) != 3 || data[0] != 'Z' {
+		return fmt.Errorf("bad quoted string")
+	}
+	i, err := strconv.ParseInt(string(data[1:3]), 16, 8)
+	if err != nil {
+		return fmt.Errorf("bad hex")
+	}
+	*b = intWithMarshalText(i)
+	return nil
+}
+
+type intWithPtrMarshalText int
+
+func (b *intWithPtrMarshalText) MarshalText() ([]byte, error) {
+	return intWithMarshalText(*b).MarshalText()
+}
+
+func (b *intWithPtrMarshalText) UnmarshalText(data []byte) error {
+	return (*intWithMarshalText)(b).UnmarshalText(data)
+}
+
+type mapStringToStringData struct {
+	Data map[string]string `json:"data"`
+}
+
+type unmarshalTest struct {
+	in                    string
+	ptr                   interface{}
+	out                   interface{}
+	err                   error
+	useNumber             bool
+	golden                bool
+	disallowUnknownFields bool
+}
+
+type B struct {
+	B bool `json:",string"`
+}
+
+var unmarshalTests = []unmarshalTest{
+	// basic types
+	{in: `true`, ptr: new(bool), out: true},
+	{in: `1`, ptr: new(int), out: 1},
+	{in: `1.2`, ptr: new(float64), out: 1.2},
+	{in: `-5`, ptr: new(int16), out: int16(-5)},
+	{in: `2`, ptr: new(Number), out: Number("2"), useNumber: true},
+	{in: `2`, ptr: new(Number), out: Number("2")},
+	{in: `2`, ptr: new(interface{}), out: float64(2.0)},
+	{in: `2`, ptr: new(interface{}), out: Number("2"), useNumber: true},
+	{in: `"a\u1234"`, ptr: new(string), out: "a\u1234"},
+	{in: `"http:\/\/"`, ptr: new(string), out: "http://"},
+	{in: `"g-clef: \uD834\uDD1E"`, ptr: new(string), out: "g-clef: \U0001D11E"},
+	{in: `"invalid: \uD834x\uDD1E"`, ptr: new(string), out: "invalid: \uFFFDx\uFFFD"},
+	{in: "null", ptr: new(interface{}), out: nil},
+	{in: `{"X": [1,2,3], "Y": 4}`, ptr: new(T), out: T{Y: 4}, err: &UnmarshalTypeError{"array", reflect.TypeOf(""), 7, "T", "X"}},
+	{in: `{"X": 23}`, ptr: new(T), out: T{}, err: &UnmarshalTypeError{"number", reflect.TypeOf(""), 8, "T", "X"}}, {in: `{"x": 1}`, ptr: new(tx), out: tx{}},
+	{in: `{"x": 1}`, ptr: new(tx), out: tx{}},
+	{in: `{"x": 1}`, ptr: new(tx), err: fmt.Errorf("json: unknown field \"x\""), disallowUnknownFields: true},
+	{in: `{"S": 23}`, ptr: new(W), out: W{}, err: &UnmarshalTypeError{"number", reflect.TypeOf(SS("")), 0, "W", "S"}},
+	{in: `{"F1":1,"F2":2,"F3":3}`, ptr: new(V), out: V{F1: float64(1), F2: int32(2), F3: Number("3")}},
+	{in: `{"F1":1,"F2":2,"F3":3}`, ptr: new(V), out: V{F1: Number("1"), F2: int32(2), F3: Number("3")}, useNumber: true},
+	{in: `{"k1":1,"k2":"s","k3":[1,2.0,3e-3],"k4":{"kk1":"s","kk2":2}}`, ptr: new(interface{}), out: ifaceNumAsFloat64},
+	{in: `{"k1":1,"k2":"s","k3":[1,2.0,3e-3],"k4":{"kk1":"s","kk2":2}}`, ptr: new(interface{}), out: ifaceNumAsNumber, useNumber: true},
+
+	// raw values with whitespace
+	{in: "\n true ", ptr: new(bool), out: true},
+	{in: "\t 1 ", ptr: new(int), out: 1},
+	{in: "\r 1.2 ", ptr: new(float64), out: 1.2},
+	{in: "\t -5 \n", ptr: new(int16), out: int16(-5)},
+	{in: "\t \"a\\u1234\" \n", ptr: new(string), out: "a\u1234"},
+
+	// Z has a "-" tag.
+	{in: `{"Y": 1, "Z": 2}`, ptr: new(T), out: T{Y: 1}},
+	{in: `{"Y": 1, "Z": 2}`, ptr: new(T), err: fmt.Errorf("json: unknown field \"Z\""), disallowUnknownFields: true},
+
+	{in: `{"alpha": "abc", "alphabet": "xyz"}`, ptr: new(U), out: U{Alphabet: "abc"}},
+	{in: `{"alpha": "abc", "alphabet": "xyz"}`, ptr: new(U), err: fmt.Errorf("json: unknown field \"alphabet\""), disallowUnknownFields: true},
+	{in: `{"alpha": "abc"}`, ptr: new(U), out: U{Alphabet: "abc"}},
+	{in: `{"alphabet": "xyz"}`, ptr: new(U), out: U{}},
+	{in: `{"alphabet": "xyz"}`, ptr: new(U), err: fmt.Errorf("json: unknown field \"alphabet\""), disallowUnknownFields: true},
+
+	// syntax errors
+	{in: `{"X": "foo", "Y"}`, err: &SyntaxError{"invalid character '}' after object key", 17}},
+	{in: `[1, 2, 3+]`, err: &SyntaxError{"invalid character '+' after array element", 9}},
+	{in: `{"X":12x}`, err: &SyntaxError{"invalid character 'x' after object key:value pair", 8}, useNumber: true},
+	{in: `[2, 3`, err: &SyntaxError{msg: "unexpected end of JSON input", Offset: 5}},
+
+	// raw value errors
+	{in: "\x01 42", err: &SyntaxError{"invalid character '\\x01' looking for beginning of value", 1}},
+	{in: " 42 \x01", err: &SyntaxError{"invalid character '\\x01' after top-level value", 5}},
+	{in: "\x01 true", err: &SyntaxError{"invalid character '\\x01' looking for beginning of value", 1}},
+	{in: " false \x01", err: &SyntaxError{"invalid character '\\x01' after top-level value", 8}},
+	{in: "\x01 1.2", err: &SyntaxError{"invalid character '\\x01' looking for beginning of value", 1}},
+	{in: " 3.4 \x01", err: &SyntaxError{"invalid character '\\x01' after top-level value", 6}},
+	{in: "\x01 \"string\"", err: &SyntaxError{"invalid character '\\x01' looking for beginning of value", 1}},
+	{in: " \"string\" \x01", err: &SyntaxError{"invalid character '\\x01' after top-level value", 11}},
+
+	// array tests
+	{in: `[1, 2, 3]`, ptr: new([3]int), out: [3]int{1, 2, 3}},
+	{in: `[1, 2, 3]`, ptr: new([1]int), out: [1]int{1}},
+	{in: `[1, 2, 3]`, ptr: new([5]int), out: [5]int{1, 2, 3, 0, 0}},
+	{in: `[1, 2, 3]`, ptr: new(MustNotUnmarshalJSON), err: errors.New("MustNotUnmarshalJSON was used")},
+
+	// empty array to interface test
+	{in: `[]`, ptr: new([]interface{}), out: []interface{}{}},
+	{in: `null`, ptr: new([]interface{}), out: []interface{}(nil)},
+	{in: `{"T":[]}`, ptr: new(map[string]interface{}), out: map[string]interface{}{"T": []interface{}{}}},
+	{in: `{"T":null}`, ptr: new(map[string]interface{}), out: map[string]interface{}{"T": interface{}(nil)}},
+
+	// composite tests
+	{in: allValueIndent, ptr: new(All), out: allValue},
+	{in: allValueCompact, ptr: new(All), out: allValue},
+	{in: allValueIndent, ptr: new(*All), out: &allValue},
+	{in: allValueCompact, ptr: new(*All), out: &allValue},
+	{in: pallValueIndent, ptr: new(All), out: pallValue},
+	{in: pallValueCompact, ptr: new(All), out: pallValue},
+	{in: pallValueIndent, ptr: new(*All), out: &pallValue},
+	{in: pallValueCompact, ptr: new(*All), out: &pallValue},
+
+	// unmarshal interface test
+	{in: `{"T":false}`, ptr: &um0, out: umtrue}, // use "false" so test will fail if custom unmarshaler is not called
+	{in: `{"T":false}`, ptr: &ump, out: &umtrue},
+	{in: `[{"T":false}]`, ptr: &umslice, out: umslice},
+	{in: `[{"T":false}]`, ptr: &umslicep, out: &umslice},
+	{in: `{"M":{"T":"x:y"}}`, ptr: &umstruct, out: umstruct},
+
+	// UnmarshalText interface test
+	{in: `"x:y"`, ptr: &um0T, out: umtrueXY},
+	{in: `"x:y"`, ptr: &umpType, out: &umtrueXY},
+	{in: `["x:y"]`, ptr: &umsliceXY, out: umsliceXY},
+	{in: `["x:y"]`, ptr: &umslicepType, out: &umsliceXY},
+	{in: `{"M":"x:y"}`, ptr: umstructType, out: umstructXY},
+
+	// integer-keyed map test
+	{
+		in:  `{"-1":"a","0":"b","1":"c"}`,
+		ptr: new(map[int]string),
+		out: map[int]string{-1: "a", 0: "b", 1: "c"},
+	},
+	{
+		in:  `{"0":"a","10":"c","9":"b"}`,
+		ptr: new(map[u8]string),
+		out: map[u8]string{0: "a", 9: "b", 10: "c"},
+	},
+	{
+		in:  `{"-9223372036854775808":"min","9223372036854775807":"max"}`,
+		ptr: new(map[int64]string),
+		out: map[int64]string{math.MinInt64: "min", math.MaxInt64: "max"},
+	},
+	{
+		in:  `{"18446744073709551615":"max"}`,
+		ptr: new(map[uint64]string),
+		out: map[uint64]string{math.MaxUint64: "max"},
+	},
+	{
+		in:  `{"0":false,"10":true}`,
+		ptr: new(map[uintptr]bool),
+		out: map[uintptr]bool{0: false, 10: true},
+	},
+
+	// Check that MarshalText and UnmarshalText take precedence
+	// over default integer handling in map keys.
+	{
+		in:  `{"u2":4}`,
+		ptr: new(map[u8marshal]int),
+		out: map[u8marshal]int{2: 4},
+	},
+	{
+		in:  `{"2":4}`,
+		ptr: new(map[u8marshal]int),
+		err: errMissingU8Prefix,
+	},
+
+	// integer-keyed map errors
+	{
+		in:  `{"abc":"abc"}`,
+		ptr: new(map[int]string),
+		err: &UnmarshalTypeError{Value: "number abc", Type: reflect.TypeOf(0), Offset: 2},
+	},
+	{
+		in:  `{"256":"abc"}`,
+		ptr: new(map[uint8]string),
+		err: &UnmarshalTypeError{Value: "number 256", Type: reflect.TypeOf(uint8(0)), Offset: 2},
+	},
+	{
+		in:  `{"128":"abc"}`,
+		ptr: new(map[int8]string),
+		err: &UnmarshalTypeError{Value: "number 128", Type: reflect.TypeOf(int8(0)), Offset: 2},
+	},
+	{
+		in:  `{"-1":"abc"}`,
+		ptr: new(map[uint8]string),
+		err: &UnmarshalTypeError{Value: "number -1", Type: reflect.TypeOf(uint8(0)), Offset: 2},
+	},
+	{
+		in:  `{"F":{"a":2,"3":4}}`,
+		ptr: new(map[string]map[int]int),
+		err: &UnmarshalTypeError{Value: "number a", Type: reflect.TypeOf(int(0)), Offset: 7},
+	},
+	{
+		in:  `{"F":{"a":2,"3":4}}`,
+		ptr: new(map[string]map[uint]int),
+		err: &UnmarshalTypeError{Value: "number a", Type: reflect.TypeOf(uint(0)), Offset: 7},
+	},
+
+	// Map keys can be encoding.TextUnmarshalers.
+	{in: `{"x:y":true}`, ptr: &ummapType, out: ummapXY},
+	// If multiple values for the same key exists, only the most recent value is used.
+	{in: `{"x:y":false,"x:y":true}`, ptr: &ummapType, out: ummapXY},
+
+	// Overwriting of data.
+	// This is different from package xml, but it's what we've always done.
+	// Now documented and tested.
+	{in: `[2]`, ptr: sliceAddr([]int{1}), out: []int{2}},
+	{in: `{"key": 2}`, ptr: mapAddr(map[string]int{"old": 0, "key": 1}), out: map[string]int{"key": 2}},
+
+	{
+		in: `{
+			"Level0": 1,
+			"Level1b": 2,
+			"Level1c": 3,
+			"x": 4,
+			"Level1a": 5,
+			"LEVEL1B": 6,
+			"e": {
+				"Level1a": 8,
+				"Level1b": 9,
+				"Level1c": 10,
+				"Level1d": 11,
+				"x": 12
+			},
+			"Loop1": 13,
+			"Loop2": 14,
+			"X": 15,
+			"Y": 16,
+			"Z": 17,
+			"Q": 18
+		}`,
+		ptr: new(Top),
+		out: Top{
+			Level0: 1,
+			Embed0: Embed0{
+				Level1b: 2,
+				Level1c: 3,
+			},
+			Embed0a: &Embed0a{
+				Level1a: 5,
+				Level1b: 6,
+			},
+			Embed0b: &Embed0b{
+				Level1a: 8,
+				Level1b: 9,
+				Level1c: 10,
+				Level1d: 11,
+				Level1e: 12,
+			},
+			Loop: Loop{
+				Loop1: 13,
+				Loop2: 14,
+			},
+			Embed0p: Embed0p{
+				Point: image.Point{X: 15, Y: 16},
+			},
+			Embed0q: Embed0q{
+				Point: Point{Z: 17},
+			},
+			embed: embed{
+				Q: 18,
+			},
+		},
+	},
+	{
+		in:  `{"hello": 1}`,
+		ptr: new(Ambig),
+		out: Ambig{First: 1},
+	},
+
+	{
+		in:  `{"X": 1,"Y":2}`,
+		ptr: new(S5),
+		out: S5{S8: S8{S9: S9{Y: 2}}},
+	},
+	{
+		in:                    `{"X": 1,"Y":2}`,
+		ptr:                   new(S5),
+		err:                   fmt.Errorf("json: unknown field \"X\""),
+		disallowUnknownFields: true,
+	},
+	{
+		in:  `{"X": 1,"Y":2}`,
+		ptr: new(S10),
+		out: S10{S13: S13{S8: S8{S9: S9{Y: 2}}}},
+	},
+	{
+		in:                    `{"X": 1,"Y":2}`,
+		ptr:                   new(S10),
+		err:                   fmt.Errorf("json: unknown field \"X\""),
+		disallowUnknownFields: true,
+	},
+
+	// invalid UTF-8 is coerced to valid UTF-8.
+	{
+		in:  "\"hello\xffworld\"",
+		ptr: new(string),
+		out: "hello\ufffdworld",
+	},
+	{
+		in:  "\"hello\xc2\xc2world\"",
+		ptr: new(string),
+		out: "hello\ufffd\ufffdworld",
+	},
+	{
+		in:  "\"hello\xc2\xffworld\"",
+		ptr: new(string),
+		out: "hello\ufffd\ufffdworld",
+	},
+	{
+		in:  "\"hello\\ud800world\"",
+		ptr: new(string),
+		out: "hello\ufffdworld",
+	},
+	{
+		in:  "\"hello\\ud800\\ud800world\"",
+		ptr: new(string),
+		out: "hello\ufffd\ufffdworld",
+	},
+	{
+		in:  "\"hello\\ud800\\ud800world\"",
+		ptr: new(string),
+		out: "hello\ufffd\ufffdworld",
+	},
+	{
+		in:  "\"hello\xed\xa0\x80\xed\xb0\x80world\"",
+		ptr: new(string),
+		out: "hello\ufffd\ufffd\ufffd\ufffd\ufffd\ufffdworld",
+	},
+
+	// Used to be issue 8305, but time.Time implements encoding.TextUnmarshaler so this works now.
+	{
+		in:  `{"2009-11-10T23:00:00Z": "hello world"}`,
+		ptr: &map[time.Time]string{},
+		out: map[time.Time]string{time.Date(2009, 11, 10, 23, 0, 0, 0, time.UTC): "hello world"},
+	},
+
+	// issue 8305
+	{
+		in:  `{"2009-11-10T23:00:00Z": "hello world"}`,
+		ptr: &map[Point]string{},
+		err: &UnmarshalTypeError{Value: "object", Type: reflect.TypeOf(map[Point]string{}), Offset: 1},
+	},
+	{
+		in:  `{"asdf": "hello world"}`,
+		ptr: &map[unmarshaler]string{},
+		err: &UnmarshalTypeError{Value: "object", Type: reflect.TypeOf(map[unmarshaler]string{}), Offset: 1},
+	},
+
+	// related to issue 13783.
+	// Go 1.7 changed marshaling a slice of typed byte to use the methods on the byte type,
+	// similar to marshaling a slice of typed int.
+	// These tests check that, assuming the byte type also has valid decoding methods,
+	// either the old base64 string encoding or the new per-element encoding can be
+	// successfully unmarshaled. The custom unmarshalers were accessible in earlier
+	// versions of Go, even though the custom marshaler was not.
+	{
+		in:  `"AQID"`,
+		ptr: new([]byteWithMarshalJSON),
+		out: []byteWithMarshalJSON{1, 2, 3},
+	},
+	{
+		in:     `["Z01","Z02","Z03"]`,
+		ptr:    new([]byteWithMarshalJSON),
+		out:    []byteWithMarshalJSON{1, 2, 3},
+		golden: true,
+	},
+	{
+		in:  `"AQID"`,
+		ptr: new([]byteWithMarshalText),
+		out: []byteWithMarshalText{1, 2, 3},
+	},
+	{
+		in:     `["Z01","Z02","Z03"]`,
+		ptr:    new([]byteWithMarshalText),
+		out:    []byteWithMarshalText{1, 2, 3},
+		golden: true,
+	},
+	{
+		in:  `"AQID"`,
+		ptr: new([]byteWithPtrMarshalJSON),
+		out: []byteWithPtrMarshalJSON{1, 2, 3},
+	},
+	{
+		in:     `["Z01","Z02","Z03"]`,
+		ptr:    new([]byteWithPtrMarshalJSON),
+		out:    []byteWithPtrMarshalJSON{1, 2, 3},
+		golden: true,
+	},
+	{
+		in:  `"AQID"`,
+		ptr: new([]byteWithPtrMarshalText),
+		out: []byteWithPtrMarshalText{1, 2, 3},
+	},
+	{
+		in:     `["Z01","Z02","Z03"]`,
+		ptr:    new([]byteWithPtrMarshalText),
+		out:    []byteWithPtrMarshalText{1, 2, 3},
+		golden: true,
+	},
+
+	// ints work with the marshaler but not the base64 []byte case
+	{
+		in:     `["Z01","Z02","Z03"]`,
+		ptr:    new([]intWithMarshalJSON),
+		out:    []intWithMarshalJSON{1, 2, 3},
+		golden: true,
+	},
+	{
+		in:     `["Z01","Z02","Z03"]`,
+		ptr:    new([]intWithMarshalText),
+		out:    []intWithMarshalText{1, 2, 3},
+		golden: true,
+	},
+	{
+		in:     `["Z01","Z02","Z03"]`,
+		ptr:    new([]intWithPtrMarshalJSON),
+		out:    []intWithPtrMarshalJSON{1, 2, 3},
+		golden: true,
+	},
+	{
+		in:     `["Z01","Z02","Z03"]`,
+		ptr:    new([]intWithPtrMarshalText),
+		out:    []intWithPtrMarshalText{1, 2, 3},
+		golden: true,
+	},
+
+	{in: `0.000001`, ptr: new(float64), out: 0.000001, golden: true},
+	{in: `1e-7`, ptr: new(float64), out: 1e-7, golden: true},
+	{in: `100000000000000000000`, ptr: new(float64), out: 100000000000000000000.0, golden: true},
+	{in: `1e+21`, ptr: new(float64), out: 1e21, golden: true},
+	{in: `-0.000001`, ptr: new(float64), out: -0.000001, golden: true},
+	{in: `-1e-7`, ptr: new(float64), out: -1e-7, golden: true},
+	{in: `-100000000000000000000`, ptr: new(float64), out: -100000000000000000000.0, golden: true},
+	{in: `-1e+21`, ptr: new(float64), out: -1e21, golden: true},
+	{in: `999999999999999900000`, ptr: new(float64), out: 999999999999999900000.0, golden: true},
+	{in: `9007199254740992`, ptr: new(float64), out: 9007199254740992.0, golden: true},
+	{in: `9007199254740993`, ptr: new(float64), out: 9007199254740992.0, golden: false},
+
+	{
+		in:  `{"V": {"F2": "hello"}}`,
+		ptr: new(VOuter),
+		err: &UnmarshalTypeError{
+			Value:  "string",
+			Struct: "V",
+			Field:  "F2",
+			Type:   reflect.TypeOf(int32(0)),
+			Offset: 20,
+		},
+	},
+	{
+		in:  `{"V": {"F4": {}, "F2": "hello"}}`,
+		ptr: new(VOuter),
+		err: &UnmarshalTypeError{
+			Value:  "string",
+			Struct: "V",
+			Field:  "F2",
+			Type:   reflect.TypeOf(int32(0)),
+			Offset: 30,
+		},
+	},
+
+	// issue 15146.
+	// invalid inputs in wrongStringTests below.
+	{in: `{"B":"true"}`, ptr: new(B), out: B{true}, golden: true},
+	{in: `{"B":"false"}`, ptr: new(B), out: B{false}, golden: true},
+	{in: `{"B": "maybe"}`, ptr: new(B), err: errors.New(`json: invalid use of ,string struct tag, trying to unmarshal "maybe" into bool`)},
+	{in: `{"B": "tru"}`, ptr: new(B), err: errors.New(`json: invalid use of ,string struct tag, trying to unmarshal "tru" into bool`)},
+	{in: `{"B": "False"}`, ptr: new(B), err: errors.New(`json: invalid use of ,string struct tag, trying to unmarshal "False" into bool`)},
+	{in: `{"B": "null"}`, ptr: new(B), out: B{false}},
+	{in: `{"B": "nul"}`, ptr: new(B), err: errors.New(`json: invalid use of ,string struct tag, trying to unmarshal "nul" into bool`)},
+	{in: `{"B": [2, 3]}`, ptr: new(B), err: errors.New(`json: invalid use of ,string struct tag, trying to unmarshal unquoted value into bool`)},
+
+	// additional tests for disallowUnknownFields
+	{
+		in: `{
+			"Level0": 1,
+			"Level1b": 2,
+			"Level1c": 3,
+			"x": 4,
+			"Level1a": 5,
+			"LEVEL1B": 6,
+			"e": {
+				"Level1a": 8,
+				"Level1b": 9,
+				"Level1c": 10,
+				"Level1d": 11,
+				"x": 12
+			},
+			"Loop1": 13,
+			"Loop2": 14,
+			"X": 15,
+			"Y": 16,
+			"Z": 17,
+			"Q": 18,
+			"extra": true
+		}`,
+		ptr:                   new(Top),
+		err:                   fmt.Errorf("json: unknown field \"extra\""),
+		disallowUnknownFields: true,
+	},
+	{
+		in: `{
+			"Level0": 1,
+			"Level1b": 2,
+			"Level1c": 3,
+			"x": 4,
+			"Level1a": 5,
+			"LEVEL1B": 6,
+			"e": {
+				"Level1a": 8,
+				"Level1b": 9,
+				"Level1c": 10,
+				"Level1d": 11,
+				"x": 12,
+				"extra": null
+			},
+			"Loop1": 13,
+			"Loop2": 14,
+			"X": 15,
+			"Y": 16,
+			"Z": 17,
+			"Q": 18
+		}`,
+		ptr:                   new(Top),
+		err:                   fmt.Errorf("json: unknown field \"extra\""),
+		disallowUnknownFields: true,
+	},
+	// issue 26444
+	// UnmarshalTypeError without field & struct values
+	{
+		in:  `{"data":{"test1": "bob", "test2": 123}}`,
+		ptr: new(mapStringToStringData),
+		err: &UnmarshalTypeError{Value: "number", Type: reflect.TypeOf(""), Offset: 37, Struct: "mapStringToStringData", Field: "data"},
+	},
+	{
+		in:  `{"data":{"test1": 123, "test2": "bob"}}`,
+		ptr: new(mapStringToStringData),
+		err: &UnmarshalTypeError{Value: "number", Type: reflect.TypeOf(""), Offset: 21, Struct: "mapStringToStringData", Field: "data"},
+	},
+
+	// trying to decode JSON arrays or objects via TextUnmarshaler
+	{
+		in:  `[1, 2, 3]`,
+		ptr: new(MustNotUnmarshalText),
+		err: &UnmarshalTypeError{Value: "array", Type: reflect.TypeOf(&MustNotUnmarshalText{}), Offset: 1},
+	},
+	{
+		in:  `{"foo": "bar"}`,
+		ptr: new(MustNotUnmarshalText),
+		err: &UnmarshalTypeError{Value: "object", Type: reflect.TypeOf(&MustNotUnmarshalText{}), Offset: 1},
+	},
+}
+
+func TestMarshal(t *testing.T) {
+	b, err := Marshal(allValue)
+	if err != nil {
+		t.Fatalf("Marshal allValue: %v", err)
+	}
+	if string(b) != allValueCompact {
+		t.Errorf("Marshal allValueCompact")
+		diff(t, b, []byte(allValueCompact))
+		return
+	}
+
+	b, err = Marshal(pallValue)
+	if err != nil {
+		t.Fatalf("Marshal pallValue: %v", err)
+	}
+	if string(b) != pallValueCompact {
+		t.Errorf("Marshal pallValueCompact")
+		diff(t, b, []byte(pallValueCompact))
+		return
+	}
+}
+
+var badUTF8 = []struct {
+	in, out string
+}{
+	{"hello\xffworld", `"hello\ufffdworld"`},
+	{"", `""`},
+	{"\xff", `"\ufffd"`},
+	{"\xff\xff", `"\ufffd\ufffd"`},
+	{"a\xffb", `"a\ufffdb"`},
+	{"\xe6\x97\xa5\xe6\x9c\xac\xff\xaa\x9e", `"日本\ufffd\ufffd\ufffd"`},
+}
+
+func TestMarshalBadUTF8(t *testing.T) {
+	for _, tt := range badUTF8 {
+		b, err := Marshal(tt.in)
+		if string(b) != tt.out || err != nil {
+			t.Errorf("Marshal(%q) = %#q, %v, want %#q, nil", tt.in, b, err, tt.out)
+		}
+	}
+}
+
+func TestMarshalNumberZeroVal(t *testing.T) {
+	var n Number
+	out, err := Marshal(n)
+	if err != nil {
+		t.Fatal(err)
+	}
+	outStr := string(out)
+	if outStr != "0" {
+		t.Fatalf("Invalid zero val for Number: %q", outStr)
+	}
+}
+
+func TestMarshalEmbeds(t *testing.T) {
+	top := &Top{
+		Level0: 1,
+		Embed0: Embed0{
+			Level1b: 2,
+			Level1c: 3,
+		},
+		Embed0a: &Embed0a{
+			Level1a: 5,
+			Level1b: 6,
+		},
+		Embed0b: &Embed0b{
+			Level1a: 8,
+			Level1b: 9,
+			Level1c: 10,
+			Level1d: 11,
+			Level1e: 12,
+		},
+		Loop: Loop{
+			Loop1: 13,
+			Loop2: 14,
+		},
+		Embed0p: Embed0p{
+			Point: image.Point{X: 15, Y: 16},
+		},
+		Embed0q: Embed0q{
+			Point: Point{Z: 17},
+		},
+		embed: embed{
+			Q: 18,
+		},
+	}
+	b, err := Marshal(top)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := "{\"Level0\":1,\"Level1b\":2,\"Level1c\":3,\"Level1a\":5,\"LEVEL1B\":6,\"e\":{\"Level1a\":8,\"Level1b\":9,\"Level1c\":10,\"Level1d\":11,\"x\":12},\"Loop1\":13,\"Loop2\":14,\"X\":15,\"Y\":16,\"Z\":17,\"Q\":18}"
+	if string(b) != want {
+		t.Errorf("Wrong marshal result.\n got: %q\nwant: %q", b, want)
+	}
+}
+
+func TestUnmarshal(t *testing.T) {
+	for i, tt := range unmarshalTests {
+		var scan scanner
+		in := []byte(tt.in)
+		if err := checkValid(in, &scan); err != nil {
+			if !reflect.DeepEqual(err, tt.err) {
+				t.Errorf("#%d: checkValid: %#v", i, err)
+				continue
+			}
+		}
+		if tt.ptr == nil {
+			continue
+		}
+
+		// v = new(right-type)
+		v := reflect.New(reflect.TypeOf(tt.ptr).Elem())
+		dec := NewDecoder(bytes.NewReader(in))
+		if tt.useNumber {
+			dec.UseNumber()
+		}
+		if tt.disallowUnknownFields {
+			dec.DisallowUnknownFields()
+		}
+		if err := dec.Decode(v.Interface()); !reflect.DeepEqual(err, tt.err) {
+			t.Errorf("#%d: %v, want %v", i, err, tt.err)
+			continue
+		} else if err != nil {
+			continue
+		}
+		if !reflect.DeepEqual(v.Elem().Interface(), tt.out) {
+			t.Errorf("#%d: mismatch\nhave: %#+v\nwant: %#+v", i, v.Elem().Interface(), tt.out)
+			data, _ := Marshal(v.Elem().Interface())
+			println(string(data))
+			data, _ = Marshal(tt.out)
+			println(string(data))
+			continue
+		}
+
+		// Check round trip also decodes correctly.
+		if tt.err == nil {
+			enc, err := Marshal(v.Interface())
+			if err != nil {
+				t.Errorf("#%d: error re-marshaling: %v", i, err)
+				continue
+			}
+			if tt.golden && !bytes.Equal(enc, in) {
+				t.Errorf("#%d: remarshal mismatch:\nhave: %s\nwant: %s", i, enc, in)
+			}
+			vv := reflect.New(reflect.TypeOf(tt.ptr).Elem())
+			dec = NewDecoder(bytes.NewReader(enc))
+			if tt.useNumber {
+				dec.UseNumber()
+			}
+			if err := dec.Decode(vv.Interface()); err != nil {
+				t.Errorf("#%d: error re-unmarshaling %#q: %v", i, enc, err)
+				continue
+			}
+			if !reflect.DeepEqual(v.Elem().Interface(), vv.Elem().Interface()) {
+				t.Errorf("#%d: mismatch\nhave: %#+v\nwant: %#+v", i, v.Elem().Interface(), vv.Elem().Interface())
+				t.Errorf("     In: %q", strings.Map(noSpace, string(in)))
+				t.Errorf("Marshal: %q", strings.Map(noSpace, string(enc)))
+				continue
+			}
+		}
+	}
+}
+
+func TestUnmarshalMarshal(t *testing.T) {
+	initBig()
+	var v interface{}
+	if err := Unmarshal(jsonBig, &v); err != nil {
+		t.Fatalf("Unmarshal: %v", err)
+	}
+	b, err := Marshal(v)
+	if err != nil {
+		t.Fatalf("Marshal: %v", err)
+	}
+	if !bytes.Equal(jsonBig, b) {
+		t.Errorf("Marshal jsonBig")
+		diff(t, b, jsonBig)
+		return
+	}
+}
+
+var numberTests = []struct {
+	in       string
+	i        int64
+	intErr   string
+	f        float64
+	floatErr string
+}{
+	{in: "-1.23e1", intErr: "strconv.ParseInt: parsing \"-1.23e1\": invalid syntax", f: -1.23e1},
+	{in: "-12", i: -12, f: -12.0},
+	{in: "1e1000", intErr: "strconv.ParseInt: parsing \"1e1000\": invalid syntax", floatErr: "strconv.ParseFloat: parsing \"1e1000\": value out of range"},
+}
+
+// Independent of Decode, basic coverage of the accessors in Number
+func TestNumberAccessors(t *testing.T) {
+	for _, tt := range numberTests {
+		n := Number(tt.in)
+		if s := n.String(); s != tt.in {
+			t.Errorf("Number(%q).String() is %q", tt.in, s)
+		}
+		if i, err := n.Int64(); err == nil && tt.intErr == "" && i != tt.i {
+			t.Errorf("Number(%q).Int64() is %d", tt.in, i)
+		} else if (err == nil && tt.intErr != "") || (err != nil && err.Error() != tt.intErr) {
+			t.Errorf("Number(%q).Int64() wanted error %q but got: %v", tt.in, tt.intErr, err)
+		}
+		if f, err := n.Float64(); err == nil && tt.floatErr == "" && f != tt.f {
+			t.Errorf("Number(%q).Float64() is %g", tt.in, f)
+		} else if (err == nil && tt.floatErr != "") || (err != nil && err.Error() != tt.floatErr) {
+			t.Errorf("Number(%q).Float64() wanted error %q but got: %v", tt.in, tt.floatErr, err)
+		}
+	}
+}
+
+func TestLargeByteSlice(t *testing.T) {
+	s0 := make([]byte, 2000)
+	for i := range s0 {
+		s0[i] = byte(i)
+	}
+	b, err := Marshal(s0)
+	if err != nil {
+		t.Fatalf("Marshal: %v", err)
+	}
+	var s1 []byte
+	if err := Unmarshal(b, &s1); err != nil {
+		t.Fatalf("Unmarshal: %v", err)
+	}
+	if !bytes.Equal(s0, s1) {
+		t.Errorf("Marshal large byte slice")
+		diff(t, s0, s1)
+	}
+}
+
+type Xint struct {
+	X int
+}
+
+func TestUnmarshalInterface(t *testing.T) {
+	var xint Xint
+	var i interface{} = &xint
+	if err := Unmarshal([]byte(`{"X":1}`), &i); err != nil {
+		t.Fatalf("Unmarshal: %v", err)
+	}
+	if xint.X != 1 {
+		t.Fatalf("Did not write to xint")
+	}
+}
+
+func TestUnmarshalPtrPtr(t *testing.T) {
+	var xint Xint
+	pxint := &xint
+	if err := Unmarshal([]byte(`{"X":1}`), &pxint); err != nil {
+		t.Fatalf("Unmarshal: %v", err)
+	}
+	if xint.X != 1 {
+		t.Fatalf("Did not write to xint")
+	}
+}
+
+func TestEscape(t *testing.T) {
+	const input = `"foobar"<html>` + " [\u2028 \u2029]"
+	const expected = `"\"foobar\"\u003chtml\u003e [\u2028 \u2029]"`
+	b, err := Marshal(input)
+	if err != nil {
+		t.Fatalf("Marshal error: %v", err)
+	}
+	if s := string(b); s != expected {
+		t.Errorf("Encoding of [%s]:\n got [%s]\nwant [%s]", input, s, expected)
+	}
+}
+
+// WrongString is a struct that's misusing the ,string modifier.
+type WrongString struct {
+	Message string `json:"result,string"`
+}
+
+type wrongStringTest struct {
+	in, err string
+}
+
+var wrongStringTests = []wrongStringTest{
+	{`{"result":"x"}`, `json: invalid use of ,string struct tag, trying to unmarshal "x" into string`},
+	{`{"result":"foo"}`, `json: invalid use of ,string struct tag, trying to unmarshal "foo" into string`},
+	{`{"result":"123"}`, `json: invalid use of ,string struct tag, trying to unmarshal "123" into string`},
+	{`{"result":123}`, `json: invalid use of ,string struct tag, trying to unmarshal unquoted value into string`},
+}
+
+// If people misuse the ,string modifier, the error message should be
+// helpful, telling the user that they're doing it wrong.
+func TestErrorMessageFromMisusedString(t *testing.T) {
+	for n, tt := range wrongStringTests {
+		r := strings.NewReader(tt.in)
+		var s WrongString
+		err := NewDecoder(r).Decode(&s)
+		got := fmt.Sprintf("%v", err)
+		if got != tt.err {
+			t.Errorf("%d. got err = %q, want %q", n, got, tt.err)
+		}
+	}
+}
+
+func noSpace(c rune) rune {
+	if isSpace(byte(c)) { //only used for ascii
+		return -1
+	}
+	return c
+}
+
+type All struct {
+	Bool    bool
+	Int     int
+	Int8    int8
+	Int16   int16
+	Int32   int32
+	Int64   int64
+	Uint    uint
+	Uint8   uint8
+	Uint16  uint16
+	Uint32  uint32
+	Uint64  uint64
+	Uintptr uintptr
+	Float32 float32
+	Float64 float64
+
+	Foo  string `json:"bar"`
+	Foo2 string `json:"bar2,dummyopt"`
+
+	IntStr     int64   `json:",string"`
+	UintptrStr uintptr `json:",string"`
+
+	PBool    *bool
+	PInt     *int
+	PInt8    *int8
+	PInt16   *int16
+	PInt32   *int32
+	PInt64   *int64
+	PUint    *uint
+	PUint8   *uint8
+	PUint16  *uint16
+	PUint32  *uint32
+	PUint64  *uint64
+	PUintptr *uintptr
+	PFloat32 *float32
+	PFloat64 *float64
+
+	String  string
+	PString *string
+
+	Map   map[string]Small
+	MapP  map[string]*Small
+	PMap  *map[string]Small
+	PMapP *map[string]*Small
+
+	EmptyMap map[string]Small
+	NilMap   map[string]Small
+
+	Slice   []Small
+	SliceP  []*Small
+	PSlice  *[]Small
+	PSliceP *[]*Small
+
+	EmptySlice []Small
+	NilSlice   []Small
+
+	StringSlice []string
+	ByteSlice   []byte
+
+	Small   Small
+	PSmall  *Small
+	PPSmall **Small
+
+	Interface  interface{}
+	PInterface *interface{}
+
+	unexported int
+}
+
+type Small struct {
+	Tag string
+}
+
+var allValue = All{
+	Bool:       true,
+	Int:        2,
+	Int8:       3,
+	Int16:      4,
+	Int32:      5,
+	Int64:      6,
+	Uint:       7,
+	Uint8:      8,
+	Uint16:     9,
+	Uint32:     10,
+	Uint64:     11,
+	Uintptr:    12,
+	Float32:    14.1,
+	Float64:    15.1,
+	Foo:        "foo",
+	Foo2:       "foo2",
+	IntStr:     42,
+	UintptrStr: 44,
+	String:     "16",
+	Map: map[string]Small{
+		"17": {Tag: "tag17"},
+		"18": {Tag: "tag18"},
+	},
+	MapP: map[string]*Small{
+		"19": {Tag: "tag19"},
+		"20": nil,
+	},
+	EmptyMap:    map[string]Small{},
+	Slice:       []Small{{Tag: "tag20"}, {Tag: "tag21"}},
+	SliceP:      []*Small{{Tag: "tag22"}, nil, {Tag: "tag23"}},
+	EmptySlice:  []Small{},
+	StringSlice: []string{"str24", "str25", "str26"},
+	ByteSlice:   []byte{27, 28, 29},
+	Small:       Small{Tag: "tag30"},
+	PSmall:      &Small{Tag: "tag31"},
+	Interface:   5.2,
+}
+
+var pallValue = All{
+	PBool:      &allValue.Bool,
+	PInt:       &allValue.Int,
+	PInt8:      &allValue.Int8,
+	PInt16:     &allValue.Int16,
+	PInt32:     &allValue.Int32,
+	PInt64:     &allValue.Int64,
+	PUint:      &allValue.Uint,
+	PUint8:     &allValue.Uint8,
+	PUint16:    &allValue.Uint16,
+	PUint32:    &allValue.Uint32,
+	PUint64:    &allValue.Uint64,
+	PUintptr:   &allValue.Uintptr,
+	PFloat32:   &allValue.Float32,
+	PFloat64:   &allValue.Float64,
+	PString:    &allValue.String,
+	PMap:       &allValue.Map,
+	PMapP:      &allValue.MapP,
+	PSlice:     &allValue.Slice,
+	PSliceP:    &allValue.SliceP,
+	PPSmall:    &allValue.PSmall,
+	PInterface: &allValue.Interface,
+}
+
+var allValueIndent = `{
+	"Bool": true,
+	"Int": 2,
+	"Int8": 3,
+	"Int16": 4,
+	"Int32": 5,
+	"Int64": 6,
+	"Uint": 7,
+	"Uint8": 8,
+	"Uint16": 9,
+	"Uint32": 10,
+	"Uint64": 11,
+	"Uintptr": 12,
+	"Float32": 14.1,
+	"Float64": 15.1,
+	"bar": "foo",
+	"bar2": "foo2",
+	"IntStr": "42",
+	"UintptrStr": "44",
+	"PBool": null,
+	"PInt": null,
+	"PInt8": null,
+	"PInt16": null,
+	"PInt32": null,
+	"PInt64": null,
+	"PUint": null,
+	"PUint8": null,
+	"PUint16": null,
+	"PUint32": null,
+	"PUint64": null,
+	"PUintptr": null,
+	"PFloat32": null,
+	"PFloat64": null,
+	"String": "16",
+	"PString": null,
+	"Map": {
+		"17": {
+			"Tag": "tag17"
+		},
+		"18": {
+			"Tag": "tag18"
+		}
+	},
+	"MapP": {
+		"19": {
+			"Tag": "tag19"
+		},
+		"20": null
+	},
+	"PMap": null,
+	"PMapP": null,
+	"EmptyMap": {},
+	"NilMap": null,
+	"Slice": [
+		{
+			"Tag": "tag20"
+		},
+		{
+			"Tag": "tag21"
+		}
+	],
+	"SliceP": [
+		{
+			"Tag": "tag22"
+		},
+		null,
+		{
+			"Tag": "tag23"
+		}
+	],
+	"PSlice": null,
+	"PSliceP": null,
+	"EmptySlice": [],
+	"NilSlice": null,
+	"StringSlice": [
+		"str24",
+		"str25",
+		"str26"
+	],
+	"ByteSlice": "Gxwd",
+	"Small": {
+		"Tag": "tag30"
+	},
+	"PSmall": {
+		"Tag": "tag31"
+	},
+	"PPSmall": null,
+	"Interface": 5.2,
+	"PInterface": null
+}`
+
+var allValueCompact = strings.Map(noSpace, allValueIndent)
+
+var pallValueIndent = `{
+	"Bool": false,
+	"Int": 0,
+	"Int8": 0,
+	"Int16": 0,
+	"Int32": 0,
+	"Int64": 0,
+	"Uint": 0,
+	"Uint8": 0,
+	"Uint16": 0,
+	"Uint32": 0,
+	"Uint64": 0,
+	"Uintptr": 0,
+	"Float32": 0,
+	"Float64": 0,
+	"bar": "",
+	"bar2": "",
+        "IntStr": "0",
+	"UintptrStr": "0",
+	"PBool": true,
+	"PInt": 2,
+	"PInt8": 3,
+	"PInt16": 4,
+	"PInt32": 5,
+	"PInt64": 6,
+	"PUint": 7,
+	"PUint8": 8,
+	"PUint16": 9,
+	"PUint32": 10,
+	"PUint64": 11,
+	"PUintptr": 12,
+	"PFloat32": 14.1,
+	"PFloat64": 15.1,
+	"String": "",
+	"PString": "16",
+	"Map": null,
+	"MapP": null,
+	"PMap": {
+		"17": {
+			"Tag": "tag17"
+		},
+		"18": {
+			"Tag": "tag18"
+		}
+	},
+	"PMapP": {
+		"19": {
+			"Tag": "tag19"
+		},
+		"20": null
+	},
+	"EmptyMap": null,
+	"NilMap": null,
+	"Slice": null,
+	"SliceP": null,
+	"PSlice": [
+		{
+			"Tag": "tag20"
+		},
+		{
+			"Tag": "tag21"
+		}
+	],
+	"PSliceP": [
+		{
+			"Tag": "tag22"
+		},
+		null,
+		{
+			"Tag": "tag23"
+		}
+	],
+	"EmptySlice": null,
+	"NilSlice": null,
+	"StringSlice": null,
+	"ByteSlice": null,
+	"Small": {
+		"Tag": ""
+	},
+	"PSmall": null,
+	"PPSmall": {
+		"Tag": "tag31"
+	},
+	"Interface": null,
+	"PInterface": 5.2
+}`
+
+var pallValueCompact = strings.Map(noSpace, pallValueIndent)
+
+func TestRefUnmarshal(t *testing.T) {
+	type S struct {
+		// Ref is defined in encode_test.go.
+		R0 Ref
+		R1 *Ref
+		R2 RefText
+		R3 *RefText
+	}
+	want := S{
+		R0: 12,
+		R1: new(Ref),
+		R2: 13,
+		R3: new(RefText),
+	}
+	*want.R1 = 12
+	*want.R3 = 13
+
+	var got S
+	if err := Unmarshal([]byte(`{"R0":"ref","R1":"ref","R2":"ref","R3":"ref"}`), &got); err != nil {
+		t.Fatalf("Unmarshal: %v", err)
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %+v, want %+v", got, want)
+	}
+}
+
+// Test that the empty string doesn't panic decoding when ,string is specified
+// Issue 3450
+func TestEmptyString(t *testing.T) {
+	type T2 struct {
+		Number1 int `json:",string"`
+		Number2 int `json:",string"`
+	}
+	data := `{"Number1":"1", "Number2":""}`
+	dec := NewDecoder(strings.NewReader(data))
+	var t2 T2
+	err := dec.Decode(&t2)
+	if err == nil {
+		t.Fatal("Decode: did not return error")
+	}
+	if t2.Number1 != 1 {
+		t.Fatal("Decode: did not set Number1")
+	}
+}
+
+// Test that a null for ,string is not replaced with the previous quoted string (issue 7046).
+// It should also not be an error (issue 2540, issue 8587).
+func TestNullString(t *testing.T) {
+	type T struct {
+		A int  `json:",string"`
+		B int  `json:",string"`
+		C *int `json:",string"`
+	}
+	data := []byte(`{"A": "1", "B": null, "C": null}`)
+	var s T
+	s.B = 1
+	s.C = new(int)
+	*s.C = 2
+	err := Unmarshal(data, &s)
+	if err != nil {
+		t.Fatalf("Unmarshal: %v", err)
+	}
+	if s.B != 1 || s.C != nil {
+		t.Fatalf("after Unmarshal, s.B=%d, s.C=%p, want 1, nil", s.B, s.C)
+	}
+}
+
+func intp(x int) *int {
+	p := new(int)
+	*p = x
+	return p
+}
+
+func intpp(x *int) **int {
+	pp := new(*int)
+	*pp = x
+	return pp
+}
+
+var interfaceSetTests = []struct {
+	pre  interface{}
+	json string
+	post interface{}
+}{
+	{"foo", `"bar"`, "bar"},
+	{"foo", `2`, 2.0},
+	{"foo", `true`, true},
+	{"foo", `null`, nil},
+
+	{nil, `null`, nil},
+	{new(int), `null`, nil},
+	{(*int)(nil), `null`, nil},
+	{new(*int), `null`, new(*int)},
+	{(**int)(nil), `null`, nil},
+	{intp(1), `null`, nil},
+	{intpp(nil), `null`, intpp(nil)},
+	{intpp(intp(1)), `null`, intpp(nil)},
+}
+
+func TestInterfaceSet(t *testing.T) {
+	for _, tt := range interfaceSetTests {
+		b := struct{ X interface{} }{tt.pre}
+		blob := `{"X":` + tt.json + `}`
+		if err := Unmarshal([]byte(blob), &b); err != nil {
+			t.Errorf("Unmarshal %#q: %v", blob, err)
+			continue
+		}
+		if !reflect.DeepEqual(b.X, tt.post) {
+			t.Errorf("Unmarshal %#q into %#v: X=%#v, want %#v", blob, tt.pre, b.X, tt.post)
+		}
+	}
+}
+
+type NullTest struct {
+	Bool      bool
+	Int       int
+	Int8      int8
+	Int16     int16
+	Int32     int32
+	Int64     int64
+	Uint      uint
+	Uint8     uint8
+	Uint16    uint16
+	Uint32    uint32
+	Uint64    uint64
+	Float32   float32
+	Float64   float64
+	String    string
+	PBool     *bool
+	Map       map[string]string
+	Slice     []string
+	Interface interface{}
+
+	PRaw    *RawMessage
+	PTime   *time.Time
+	PBigInt *big.Int
+	PText   *MustNotUnmarshalText
+	PBuffer *bytes.Buffer // has methods, just not relevant ones
+	PStruct *struct{}
+
+	Raw    RawMessage
+	Time   time.Time
+	BigInt big.Int
+	Text   MustNotUnmarshalText
+	Buffer bytes.Buffer
+	Struct struct{}
+}
+
+type NullTestStrings struct {
+	Bool      bool              `json:",string"`
+	Int       int               `json:",string"`
+	Int8      int8              `json:",string"`
+	Int16     int16             `json:",string"`
+	Int32     int32             `json:",string"`
+	Int64     int64             `json:",string"`
+	Uint      uint              `json:",string"`
+	Uint8     uint8             `json:",string"`
+	Uint16    uint16            `json:",string"`
+	Uint32    uint32            `json:",string"`
+	Uint64    uint64            `json:",string"`
+	Float32   float32           `json:",string"`
+	Float64   float64           `json:",string"`
+	String    string            `json:",string"`
+	PBool     *bool             `json:",string"`
+	Map       map[string]string `json:",string"`
+	Slice     []string          `json:",string"`
+	Interface interface{}       `json:",string"`
+
+	PRaw    *RawMessage           `json:",string"`
+	PTime   *time.Time            `json:",string"`
+	PBigInt *big.Int              `json:",string"`
+	PText   *MustNotUnmarshalText `json:",string"`
+	PBuffer *bytes.Buffer         `json:",string"`
+	PStruct *struct{}             `json:",string"`
+
+	Raw    RawMessage           `json:",string"`
+	Time   time.Time            `json:",string"`
+	BigInt big.Int              `json:",string"`
+	Text   MustNotUnmarshalText `json:",string"`
+	Buffer bytes.Buffer         `json:",string"`
+	Struct struct{}             `json:",string"`
+}
+
+// JSON null values should be ignored for primitives and string values instead of resulting in an error.
+// Issue 2540
+func TestUnmarshalNulls(t *testing.T) {
+	// Unmarshal docs:
+	// The JSON null value unmarshals into an interface, map, pointer, or slice
+	// by setting that Go value to nil. Because null is often used in JSON to mean
+	// ``not present,'' unmarshaling a JSON null into any other Go type has no effect
+	// on the value and produces no error.
+
+	jsonData := []byte(`{
+				"Bool"    : null,
+				"Int"     : null,
+				"Int8"    : null,
+				"Int16"   : null,
+				"Int32"   : null,
+				"Int64"   : null,
+				"Uint"    : null,
+				"Uint8"   : null,
+				"Uint16"  : null,
+				"Uint32"  : null,
+				"Uint64"  : null,
+				"Float32" : null,
+				"Float64" : null,
+				"String"  : null,
+				"PBool": null,
+				"Map": null,
+				"Slice": null,
+				"Interface": null,
+				"PRaw": null,
+				"PTime": null,
+				"PBigInt": null,
+				"PText": null,
+				"PBuffer": null,
+				"PStruct": null,
+				"Raw": null,
+				"Time": null,
+				"BigInt": null,
+				"Text": null,
+				"Buffer": null,
+				"Struct": null
+			}`)
+	nulls := NullTest{
+		Bool:      true,
+		Int:       2,
+		Int8:      3,
+		Int16:     4,
+		Int32:     5,
+		Int64:     6,
+		Uint:      7,
+		Uint8:     8,
+		Uint16:    9,
+		Uint32:    10,
+		Uint64:    11,
+		Float32:   12.1,
+		Float64:   13.1,
+		String:    "14",
+		PBool:     new(bool),
+		Map:       map[string]string{},
+		Slice:     []string{},
+		Interface: new(MustNotUnmarshalJSON),
+		PRaw:      new(RawMessage),
+		PTime:     new(time.Time),
+		PBigInt:   new(big.Int),
+		PText:     new(MustNotUnmarshalText),
+		PStruct:   new(struct{}),
+		PBuffer:   new(bytes.Buffer),
+		Raw:       RawMessage("123"),
+		Time:      time.Unix(123456789, 0),
+		BigInt:    *big.NewInt(123),
+	}
+
+	before := nulls.Time.String()
+
+	err := Unmarshal(jsonData, &nulls)
+	if err != nil {
+		t.Errorf("Unmarshal of null values failed: %v", err)
+	}
+	if !nulls.Bool || nulls.Int != 2 || nulls.Int8 != 3 || nulls.Int16 != 4 || nulls.Int32 != 5 || nulls.Int64 != 6 ||
+		nulls.Uint != 7 || nulls.Uint8 != 8 || nulls.Uint16 != 9 || nulls.Uint32 != 10 || nulls.Uint64 != 11 ||
+		nulls.Float32 != 12.1 || nulls.Float64 != 13.1 || nulls.String != "14" {
+		t.Errorf("Unmarshal of null values affected primitives")
+	}
+
+	if nulls.PBool != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.PBool")
+	}
+	if nulls.Map != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.Map")
+	}
+	if nulls.Slice != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.Slice")
+	}
+	if nulls.Interface != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.Interface")
+	}
+	if nulls.PRaw != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.PRaw")
+	}
+	if nulls.PTime != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.PTime")
+	}
+	if nulls.PBigInt != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.PBigInt")
+	}
+	if nulls.PText != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.PText")
+	}
+	if nulls.PBuffer != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.PBuffer")
+	}
+	if nulls.PStruct != nil {
+		t.Errorf("Unmarshal of null did not clear nulls.PStruct")
+	}
+
+	if string(nulls.Raw) != "null" {
+		t.Errorf("Unmarshal of RawMessage null did not record null: %v", string(nulls.Raw))
+	}
+	if nulls.Time.String() != before {
+		t.Errorf("Unmarshal of time.Time null set time to %v", nulls.Time.String())
+	}
+	if nulls.BigInt.String() != "123" {
+		t.Errorf("Unmarshal of big.Int null set int to %v", nulls.BigInt.String())
+	}
+}
+
+type MustNotUnmarshalJSON struct{}
+
+func (x MustNotUnmarshalJSON) UnmarshalJSON(data []byte) error {
+	return errors.New("MustNotUnmarshalJSON was used")
+}
+
+type MustNotUnmarshalText struct{}
+
+func (x MustNotUnmarshalText) UnmarshalText(text []byte) error {
+	return errors.New("MustNotUnmarshalText was used")
+}
+
+func TestStringKind(t *testing.T) {
+	type stringKind string
+
+	var m1, m2 map[stringKind]int
+	m1 = map[stringKind]int{
+		"foo": 42,
+	}
+
+	data, err := Marshal(m1)
+	if err != nil {
+		t.Errorf("Unexpected error marshaling: %v", err)
+	}
+
+	err = Unmarshal(data, &m2)
+	if err != nil {
+		t.Errorf("Unexpected error unmarshaling: %v", err)
+	}
+
+	if !reflect.DeepEqual(m1, m2) {
+		t.Error("Items should be equal after encoding and then decoding")
+	}
+}
+
+// Custom types with []byte as underlying type could not be marshaled
+// and then unmarshaled.
+// Issue 8962.
+func TestByteKind(t *testing.T) {
+	type byteKind []byte
+
+	a := byteKind("hello")
+
+	data, err := Marshal(a)
+	if err != nil {
+		t.Error(err)
+	}
+	var b byteKind
+	err = Unmarshal(data, &b)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(a, b) {
+		t.Errorf("expected %v == %v", a, b)
+	}
+}
+
+// The fix for issue 8962 introduced a regression.
+// Issue 12921.
+func TestSliceOfCustomByte(t *testing.T) {
+	type Uint8 uint8
+
+	a := []Uint8("hello")
+
+	data, err := Marshal(a)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var b []Uint8
+	err = Unmarshal(data, &b)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(a, b) {
+		t.Fatalf("expected %v == %v", a, b)
+	}
+}
+
+var decodeTypeErrorTests = []struct {
+	dest interface{}
+	src  string
+}{
+	{new(string), `{"user": "name"}`}, // issue 4628.
+	{new(error), `{}`},                // issue 4222
+	{new(error), `[]`},
+	{new(error), `""`},
+	{new(error), `123`},
+	{new(error), `true`},
+}
+
+func TestUnmarshalTypeError(t *testing.T) {
+	for _, item := range decodeTypeErrorTests {
+		err := Unmarshal([]byte(item.src), item.dest)
+		if _, ok := err.(*UnmarshalTypeError); !ok {
+			t.Errorf("expected type error for Unmarshal(%q, type %T): got %T",
+				item.src, item.dest, err)
+		}
+	}
+}
+
+var unmarshalSyntaxTests = []string{
+	"tru",
+	"fals",
+	"nul",
+	"123e",
+	`"hello`,
+	`[1,2,3`,
+	`{"key":1`,
+	`{"key":1,`,
+}
+
+func TestUnmarshalSyntax(t *testing.T) {
+	var x interface{}
+	for _, src := range unmarshalSyntaxTests {
+		err := Unmarshal([]byte(src), &x)
+		if _, ok := err.(*SyntaxError); !ok {
+			t.Errorf("expected syntax error for Unmarshal(%q): got %T", src, err)
+		}
+	}
+}
+
+// Test handling of unexported fields that should be ignored.
+// Issue 4660
+type unexportedFields struct {
+	Name string
+	m    map[string]interface{} `json:"-"`
+	m2   map[string]interface{} `json:"abcd"`
+
+	s []int `json:"-"`
+}
+
+func TestUnmarshalUnexported(t *testing.T) {
+	input := `{"Name": "Bob", "m": {"x": 123}, "m2": {"y": 456}, "abcd": {"z": 789}, "s": [2, 3]}`
+	want := &unexportedFields{Name: "Bob"}
+
+	out := &unexportedFields{}
+	err := Unmarshal([]byte(input), out)
+	if err != nil {
+		t.Errorf("got error %v, expected nil", err)
+	}
+	if !reflect.DeepEqual(out, want) {
+		t.Errorf("got %q, want %q", out, want)
+	}
+}
+
+// Time3339 is a time.Time which encodes to and from JSON
+// as an RFC 3339 time in UTC.
+type Time3339 time.Time
+
+func (t *Time3339) UnmarshalJSON(b []byte) error {
+	if len(b) < 2 || b[0] != '"' || b[len(b)-1] != '"' {
+		return fmt.Errorf("types: failed to unmarshal non-string value %q as an RFC 3339 time", b)
+	}
+	tm, err := time.Parse(time.RFC3339, string(b[1:len(b)-1]))
+	if err != nil {
+		return err
+	}
+	*t = Time3339(tm)
+	return nil
+}
+
+func TestUnmarshalJSONLiteralError(t *testing.T) {
+	var t3 Time3339
+	err := Unmarshal([]byte(`"0000-00-00T00:00:00Z"`), &t3)
+	if err == nil {
+		t.Fatalf("expected error; got time %v", time.Time(t3))
+	}
+	if !strings.Contains(err.Error(), "range") {
+		t.Errorf("got err = %v; want out of range error", err)
+	}
+}
+
+// Test that extra object elements in an array do not result in a
+// "data changing underfoot" error.
+// Issue 3717
+func TestSkipArrayObjects(t *testing.T) {
+	json := `[{}]`
+	var dest [0]interface{}
+
+	err := Unmarshal([]byte(json), &dest)
+	if err != nil {
+		t.Errorf("got error %q, want nil", err)
+	}
+}
+
+// Test semantics of pre-filled struct fields and pre-filled map fields.
+// Issue 4900.
+func TestPrefilled(t *testing.T) {
+	ptrToMap := func(m map[string]interface{}) *map[string]interface{} { return &m }
+
+	// Values here change, cannot reuse table across runs.
+	var prefillTests = []struct {
+		in  string
+		ptr interface{}
+		out interface{}
+	}{
+		{
+			in:  `{"X": 1, "Y": 2}`,
+			ptr: &XYZ{X: float32(3), Y: int16(4), Z: 1.5},
+			out: &XYZ{X: float64(1), Y: float64(2), Z: 1.5},
+		},
+		{
+			in:  `{"X": 1, "Y": 2}`,
+			ptr: ptrToMap(map[string]interface{}{"X": float32(3), "Y": int16(4), "Z": 1.5}),
+			out: ptrToMap(map[string]interface{}{"X": float64(1), "Y": float64(2), "Z": 1.5}),
+		},
+	}
+
+	for _, tt := range prefillTests {
+		ptrstr := fmt.Sprintf("%v", tt.ptr)
+		err := Unmarshal([]byte(tt.in), tt.ptr) // tt.ptr edited here
+		if err != nil {
+			t.Errorf("Unmarshal: %v", err)
+		}
+		if !reflect.DeepEqual(tt.ptr, tt.out) {
+			t.Errorf("Unmarshal(%#q, %s): have %v, want %v", tt.in, ptrstr, tt.ptr, tt.out)
+		}
+	}
+}
+
+var invalidUnmarshalTests = []struct {
+	v    interface{}
+	want string
+}{
+	{nil, "json: Unmarshal(nil)"},
+	{struct{}{}, "json: Unmarshal(non-pointer struct {})"},
+	{(*int)(nil), "json: Unmarshal(nil *int)"},
+}
+
+func TestInvalidUnmarshal(t *testing.T) {
+	buf := []byte(`{"a":"1"}`)
+	for _, tt := range invalidUnmarshalTests {
+		err := Unmarshal(buf, tt.v)
+		if err == nil {
+			t.Errorf("Unmarshal expecting error, got nil")
+			continue
+		}
+		if got := err.Error(); got != tt.want {
+			t.Errorf("Unmarshal = %q; want %q", got, tt.want)
+		}
+	}
+}
+
+var invalidUnmarshalTextTests = []struct {
+	v    interface{}
+	want string
+}{
+	{nil, "json: Unmarshal(nil)"},
+	{struct{}{}, "json: Unmarshal(non-pointer struct {})"},
+	{(*int)(nil), "json: Unmarshal(nil *int)"},
+	{new(net.IP), "json: cannot unmarshal number into Go value of type *net.IP"},
+}
+
+func TestInvalidUnmarshalText(t *testing.T) {
+	buf := []byte(`123`)
+	for _, tt := range invalidUnmarshalTextTests {
+		err := Unmarshal(buf, tt.v)
+		if err == nil {
+			t.Errorf("Unmarshal expecting error, got nil")
+			continue
+		}
+		if got := err.Error(); got != tt.want {
+			t.Errorf("Unmarshal = %q; want %q", got, tt.want)
+		}
+	}
+}
+
+// Test that string option is ignored for invalid types.
+// Issue 9812.
+func TestInvalidStringOption(t *testing.T) {
+	num := 0
+	item := struct {
+		T time.Time         `json:",string"`
+		M map[string]string `json:",string"`
+		S []string          `json:",string"`
+		A [1]string         `json:",string"`
+		I interface{}       `json:",string"`
+		P *int              `json:",string"`
+	}{M: make(map[string]string), S: make([]string, 0), I: num, P: &num}
+
+	data, err := Marshal(item)
+	if err != nil {
+		t.Fatalf("Marshal: %v", err)
+	}
+
+	err = Unmarshal(data, &item)
+	if err != nil {
+		t.Fatalf("Unmarshal: %v", err)
+	}
+}
+
+// Test unmarshal behavior with regards to embedded unexported structs.
+//
+// (Issue 21357) If the embedded struct is a pointer and is unallocated,
+// this returns an error because unmarshal cannot set the field.
+//
+// (Issue 24152) If the embedded struct is given an explicit name,
+// ensure that the normal unmarshal logic does not panic in reflect.
+//
+// (Issue 28145) If the embedded struct is given an explicit name and has
+// exported methods, don't cause a panic trying to get its value.
+func TestUnmarshalEmbeddedUnexported(t *testing.T) {
+	type (
+		embed1 struct{ Q int }
+		embed2 struct{ Q int }
+		embed3 struct {
+			Q int64 `json:",string"`
+		}
+		S1 struct {
+			*embed1
+			R int
+		}
+		S2 struct {
+			*embed1
+			Q int
+		}
+		S3 struct {
+			embed1
+			R int
+		}
+		S4 struct {
+			*embed1
+			embed2
+		}
+		S5 struct {
+			*embed3
+			R int
+		}
+		S6 struct {
+			embed1 `json:"embed1"`
+		}
+		S7 struct {
+			embed1 `json:"embed1"`
+			embed2
+		}
+		S8 struct {
+			embed1 `json:"embed1"`
+			embed2 `json:"embed2"`
+			Q      int
+		}
+		S9 struct {
+			unexportedWithMethods `json:"embed"`
+		}
+	)
+
+	tests := []struct {
+		in  string
+		ptr interface{}
+		out interface{}
+		err error
+	}{{
+		// Error since we cannot set S1.embed1, but still able to set S1.R.
+		in:  `{"R":2,"Q":1}`,
+		ptr: new(S1),
+		out: &S1{R: 2},
+		err: fmt.Errorf("json: cannot set embedded pointer to unexported struct: json.embed1"),
+	}, {
+		// The top level Q field takes precedence.
+		in:  `{"Q":1}`,
+		ptr: new(S2),
+		out: &S2{Q: 1},
+	}, {
+		// No issue with non-pointer variant.
+		in:  `{"R":2,"Q":1}`,
+		ptr: new(S3),
+		out: &S3{embed1: embed1{Q: 1}, R: 2},
+	}, {
+		// No error since both embedded structs have field R, which annihilate each other.
+		// Thus, no attempt is made at setting S4.embed1.
+		in:  `{"R":2}`,
+		ptr: new(S4),
+		out: new(S4),
+	}, {
+		// Error since we cannot set S5.embed1, but still able to set S5.R.
+		in:  `{"R":2,"Q":1}`,
+		ptr: new(S5),
+		out: &S5{R: 2},
+		err: fmt.Errorf("json: cannot set embedded pointer to unexported struct: json.embed3"),
+	}, {
+		// Issue 24152, ensure decodeState.indirect does not panic.
+		in:  `{"embed1": {"Q": 1}}`,
+		ptr: new(S6),
+		out: &S6{embed1{1}},
+	}, {
+		// Issue 24153, check that we can still set forwarded fields even in
+		// the presence of a name conflict.
+		//
+		// This relies on obscure behavior of reflect where it is possible
+		// to set a forwarded exported field on an unexported embedded struct
+		// even though there is a name conflict, even when it would have been
+		// impossible to do so according to Go visibility rules.
+		// Go forbids this because it is ambiguous whether S7.Q refers to
+		// S7.embed1.Q or S7.embed2.Q. Since embed1 and embed2 are unexported,
+		// it should be impossible for an external package to set either Q.
+		//
+		// It is probably okay for a future reflect change to break this.
+		in:  `{"embed1": {"Q": 1}, "Q": 2}`,
+		ptr: new(S7),
+		out: &S7{embed1{1}, embed2{2}},
+	}, {
+		// Issue 24153, similar to the S7 case.
+		in:  `{"embed1": {"Q": 1}, "embed2": {"Q": 2}, "Q": 3}`,
+		ptr: new(S8),
+		out: &S8{embed1{1}, embed2{2}, 3},
+	}, {
+		// Issue 228145, similar to the cases above.
+		in:  `{"embed": {}}`,
+		ptr: new(S9),
+		out: &S9{},
+	}}
+
+	for i, tt := range tests {
+		err := Unmarshal([]byte(tt.in), tt.ptr)
+		if !reflect.DeepEqual(err, tt.err) {
+			t.Errorf("#%d: %v, want %v", i, err, tt.err)
+		}
+		if !reflect.DeepEqual(tt.ptr, tt.out) {
+			t.Errorf("#%d: mismatch\ngot:  %#+v\nwant: %#+v", i, tt.ptr, tt.out)
+		}
+	}
+}
+
+type unmarshalPanic struct{}
+
+func (unmarshalPanic) UnmarshalJSON([]byte) error { panic(0xdead) }
+
+func TestUnmarshalPanic(t *testing.T) {
+	defer func() {
+		if got := recover(); !reflect.DeepEqual(got, 0xdead) {
+			t.Errorf("panic() = (%T)(%v), want 0xdead", got, got)
+		}
+	}()
+	Unmarshal([]byte("{}"), &unmarshalPanic{})
+	t.Fatalf("Unmarshal should have panicked")
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/encode.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/encode.go
new file mode 100644
index 0000000..dea63f1
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/encode.go
@@ -0,0 +1,1273 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package json implements encoding and decoding of JSON as defined in
+// RFC 7159. The mapping between JSON and Go values is described
+// in the documentation for the Marshal and Unmarshal functions.
+//
+// See "JSON and Go" for an introduction to this package:
+// https://golang.org/doc/articles/json_and_go.html
+package json
+
+import (
+	"bytes"
+	"encoding"
+	"encoding/base64"
+	"fmt"
+	"math"
+	"reflect"
+	"sort"
+	"strconv"
+	"strings"
+	"sync"
+	"unicode"
+	"unicode/utf8"
+)
+
+// Marshal returns the JSON encoding of v.
+//
+// Marshal traverses the value v recursively.
+// If an encountered value implements the Marshaler interface
+// and is not a nil pointer, Marshal calls its MarshalJSON method
+// to produce JSON. If no MarshalJSON method is present but the
+// value implements encoding.TextMarshaler instead, Marshal calls
+// its MarshalText method and encodes the result as a JSON string.
+// The nil pointer exception is not strictly necessary
+// but mimics a similar, necessary exception in the behavior of
+// UnmarshalJSON.
+//
+// Otherwise, Marshal uses the following type-dependent default encodings:
+//
+// Boolean values encode as JSON booleans.
+//
+// Floating point, integer, and Number values encode as JSON numbers.
+//
+// String values encode as JSON strings coerced to valid UTF-8,
+// replacing invalid bytes with the Unicode replacement rune.
+// The angle brackets "<" and ">" are escaped to "\u003c" and "\u003e"
+// to keep some browsers from misinterpreting JSON output as HTML.
+// Ampersand "&" is also escaped to "\u0026" for the same reason.
+// This escaping can be disabled using an Encoder that had SetEscapeHTML(false)
+// called on it.
+//
+// Array and slice values encode as JSON arrays, except that
+// []byte encodes as a base64-encoded string, and a nil slice
+// encodes as the null JSON value.
+//
+// Struct values encode as JSON objects.
+// Each exported struct field becomes a member of the object, using the
+// field name as the object key, unless the field is omitted for one of the
+// reasons given below.
+//
+// The encoding of each struct field can be customized by the format string
+// stored under the "json" key in the struct field's tag.
+// The format string gives the name of the field, possibly followed by a
+// comma-separated list of options. The name may be empty in order to
+// specify options without overriding the default field name.
+//
+// The "omitempty" option specifies that the field should be omitted
+// from the encoding if the field has an empty value, defined as
+// false, 0, a nil pointer, a nil interface value, and any empty array,
+// slice, map, or string.
+//
+// As a special case, if the field tag is "-", the field is always omitted.
+// Note that a field with name "-" can still be generated using the tag "-,".
+//
+// Examples of struct field tags and their meanings:
+//
+//   // Field appears in JSON as key "myName".
+//   Field int `json:"myName"`
+//
+//   // Field appears in JSON as key "myName" and
+//   // the field is omitted from the object if its value is empty,
+//   // as defined above.
+//   Field int `json:"myName,omitempty"`
+//
+//   // Field appears in JSON as key "Field" (the default), but
+//   // the field is skipped if empty.
+//   // Note the leading comma.
+//   Field int `json:",omitempty"`
+//
+//   // Field is ignored by this package.
+//   Field int `json:"-"`
+//
+//   // Field appears in JSON as key "-".
+//   Field int `json:"-,"`
+//
+// The "string" option signals that a field is stored as JSON inside a
+// JSON-encoded string. It applies only to fields of string, floating point,
+// integer, or boolean types. This extra level of encoding is sometimes used
+// when communicating with JavaScript programs:
+//
+//    Int64String int64 `json:",string"`
+//
+// The key name will be used if it's a non-empty string consisting of
+// only Unicode letters, digits, and ASCII punctuation except quotation
+// marks, backslash, and comma.
+//
+// Anonymous struct fields are usually marshaled as if their inner exported fields
+// were fields in the outer struct, subject to the usual Go visibility rules amended
+// as described in the next paragraph.
+// An anonymous struct field with a name given in its JSON tag is treated as
+// having that name, rather than being anonymous.
+// An anonymous struct field of interface type is treated the same as having
+// that type as its name, rather than being anonymous.
+//
+// The Go visibility rules for struct fields are amended for JSON when
+// deciding which field to marshal or unmarshal. If there are
+// multiple fields at the same level, and that level is the least
+// nested (and would therefore be the nesting level selected by the
+// usual Go rules), the following extra rules apply:
+//
+// 1) Of those fields, if any are JSON-tagged, only tagged fields are considered,
+// even if there are multiple untagged fields that would otherwise conflict.
+//
+// 2) If there is exactly one field (tagged or not according to the first rule), that is selected.
+//
+// 3) Otherwise there are multiple fields, and all are ignored; no error occurs.
+//
+// Handling of anonymous struct fields is new in Go 1.1.
+// Prior to Go 1.1, anonymous struct fields were ignored. To force ignoring of
+// an anonymous struct field in both current and earlier versions, give the field
+// a JSON tag of "-".
+//
+// Map values encode as JSON objects. The map's key type must either be a
+// string, an integer type, or implement encoding.TextMarshaler. The map keys
+// are sorted and used as JSON object keys by applying the following rules,
+// subject to the UTF-8 coercion described for string values above:
+//   - string keys are used directly
+//   - encoding.TextMarshalers are marshaled
+//   - integer keys are converted to strings
+//
+// Pointer values encode as the value pointed to.
+// A nil pointer encodes as the null JSON value.
+//
+// Interface values encode as the value contained in the interface.
+// A nil interface value encodes as the null JSON value.
+//
+// Channel, complex, and function values cannot be encoded in JSON.
+// Attempting to encode such a value causes Marshal to return
+// an UnsupportedTypeError.
+//
+// JSON cannot represent cyclic data structures and Marshal does not
+// handle them. Passing cyclic structures to Marshal will result in
+// an infinite recursion.
+//
+func Marshal(v interface{}) ([]byte, error) {
+	e := newEncodeState()
+
+	err := e.marshal(v, encOpts{escapeHTML: true})
+	if err != nil {
+		return nil, err
+	}
+	buf := append([]byte(nil), e.Bytes()...)
+
+	e.Reset()
+	encodeStatePool.Put(e)
+
+	return buf, nil
+}
+
+// MarshalIndent is like Marshal but applies Indent to format the output.
+// Each JSON element in the output will begin on a new line beginning with prefix
+// followed by one or more copies of indent according to the indentation nesting.
+func MarshalIndent(v interface{}, prefix, indent string) ([]byte, error) {
+	b, err := Marshal(v)
+	if err != nil {
+		return nil, err
+	}
+	var buf bytes.Buffer
+	err = Indent(&buf, b, prefix, indent)
+	if err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}
+
+// HTMLEscape appends to dst the JSON-encoded src with <, >, &, U+2028 and U+2029
+// characters inside string literals changed to \u003c, \u003e, \u0026, \u2028, \u2029
+// so that the JSON will be safe to embed inside HTML <script> tags.
+// For historical reasons, web browsers don't honor standard HTML
+// escaping within <script> tags, so an alternative JSON encoding must
+// be used.
+func HTMLEscape(dst *bytes.Buffer, src []byte) {
+	// The characters can only appear in string literals,
+	// so just scan the string one byte at a time.
+	start := 0
+	for i, c := range src {
+		if c == '<' || c == '>' || c == '&' {
+			if start < i {
+				dst.Write(src[start:i])
+			}
+			dst.WriteString(`\u00`)
+			dst.WriteByte(hex[c>>4])
+			dst.WriteByte(hex[c&0xF])
+			start = i + 1
+		}
+		// Convert U+2028 and U+2029 (E2 80 A8 and E2 80 A9).
+		if c == 0xE2 && i+2 < len(src) && src[i+1] == 0x80 && src[i+2]&^1 == 0xA8 {
+			if start < i {
+				dst.Write(src[start:i])
+			}
+			dst.WriteString(`\u202`)
+			dst.WriteByte(hex[src[i+2]&0xF])
+			start = i + 3
+		}
+	}
+	if start < len(src) {
+		dst.Write(src[start:])
+	}
+}
+
+// Marshaler is the interface implemented by types that
+// can marshal themselves into valid JSON.
+type Marshaler interface {
+	MarshalJSON() ([]byte, error)
+}
+
+// An UnsupportedTypeError is returned by Marshal when attempting
+// to encode an unsupported value type.
+type UnsupportedTypeError struct {
+	Type reflect.Type
+}
+
+func (e *UnsupportedTypeError) Error() string {
+	return "json: unsupported type: " + e.Type.String()
+}
+
+type UnsupportedValueError struct {
+	Value reflect.Value
+	Str   string
+}
+
+func (e *UnsupportedValueError) Error() string {
+	return "json: unsupported value: " + e.Str
+}
+
+// Before Go 1.2, an InvalidUTF8Error was returned by Marshal when
+// attempting to encode a string value with invalid UTF-8 sequences.
+// As of Go 1.2, Marshal instead coerces the string to valid UTF-8 by
+// replacing invalid bytes with the Unicode replacement rune U+FFFD.
+//
+// Deprecated: No longer used; kept for compatibility.
+type InvalidUTF8Error struct {
+	S string // the whole string value that caused the error
+}
+
+func (e *InvalidUTF8Error) Error() string {
+	return "json: invalid UTF-8 in string: " + strconv.Quote(e.S)
+}
+
+// A MarshalerError represents an error from calling a MarshalJSON or MarshalText method.
+type MarshalerError struct {
+	Type reflect.Type
+	Err  error
+}
+
+func (e *MarshalerError) Error() string {
+	return "json: error calling MarshalJSON for type " + e.Type.String() + ": " + e.Err.Error()
+}
+
+var hex = "0123456789abcdef"
+
+// An encodeState encodes JSON into a bytes.Buffer.
+type encodeState struct {
+	bytes.Buffer // accumulated output
+	scratch      [64]byte
+}
+
+var encodeStatePool sync.Pool
+
+func newEncodeState() *encodeState {
+	if v := encodeStatePool.Get(); v != nil {
+		e := v.(*encodeState)
+		e.Reset()
+		return e
+	}
+	return new(encodeState)
+}
+
+// jsonError is an error wrapper type for internal use only.
+// Panics with errors are wrapped in jsonError so that the top-level recover
+// can distinguish intentional panics from this package.
+type jsonError struct{ error }
+
+func (e *encodeState) marshal(v interface{}, opts encOpts) (err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			if je, ok := r.(jsonError); ok {
+				err = je.error
+			} else {
+				panic(r)
+			}
+		}
+	}()
+	e.reflectValue(reflect.ValueOf(v), opts)
+	return nil
+}
+
+// error aborts the encoding by panicking with err wrapped in jsonError.
+func (e *encodeState) error(err error) {
+	panic(jsonError{err})
+}
+
+func isEmptyValue(v reflect.Value) bool {
+	switch v.Kind() {
+	case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
+		return v.Len() == 0
+	case reflect.Bool:
+		return !v.Bool()
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		return v.Int() == 0
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		return v.Uint() == 0
+	case reflect.Float32, reflect.Float64:
+		return v.Float() == 0
+	case reflect.Interface, reflect.Ptr:
+		return v.IsNil()
+	}
+	return false
+}
+
+func (e *encodeState) reflectValue(v reflect.Value, opts encOpts) {
+	valueEncoder(v)(e, v, opts)
+}
+
+type encOpts struct {
+	// quoted causes primitive fields to be encoded inside JSON strings.
+	quoted bool
+	// escapeHTML causes '<', '>', and '&' to be escaped in JSON strings.
+	escapeHTML bool
+}
+
+type encoderFunc func(e *encodeState, v reflect.Value, opts encOpts)
+
+var encoderCache sync.Map // map[reflect.Type]encoderFunc
+
+func valueEncoder(v reflect.Value) encoderFunc {
+	if !v.IsValid() {
+		return invalidValueEncoder
+	}
+	return typeEncoder(v.Type())
+}
+
+func typeEncoder(t reflect.Type) encoderFunc {
+	if fi, ok := encoderCache.Load(t); ok {
+		return fi.(encoderFunc)
+	}
+
+	// To deal with recursive types, populate the map with an
+	// indirect func before we build it. This type waits on the
+	// real func (f) to be ready and then calls it. This indirect
+	// func is only used for recursive types.
+	var (
+		wg sync.WaitGroup
+		f  encoderFunc
+	)
+	wg.Add(1)
+	fi, loaded := encoderCache.LoadOrStore(t, encoderFunc(func(e *encodeState, v reflect.Value, opts encOpts) {
+		wg.Wait()
+		f(e, v, opts)
+	}))
+	if loaded {
+		return fi.(encoderFunc)
+	}
+
+	// Compute the real encoder and replace the indirect func with it.
+	f = newTypeEncoder(t, true)
+	wg.Done()
+	encoderCache.Store(t, f)
+	return f
+}
+
+var (
+	marshalerType     = reflect.TypeOf((*Marshaler)(nil)).Elem()
+	textMarshalerType = reflect.TypeOf((*encoding.TextMarshaler)(nil)).Elem()
+)
+
+// newTypeEncoder constructs an encoderFunc for a type.
+// The returned encoder only checks CanAddr when allowAddr is true.
+func newTypeEncoder(t reflect.Type, allowAddr bool) encoderFunc {
+	if t.Implements(marshalerType) {
+		return marshalerEncoder
+	}
+	if t.Kind() != reflect.Ptr && allowAddr {
+		if reflect.PtrTo(t).Implements(marshalerType) {
+			return newCondAddrEncoder(addrMarshalerEncoder, newTypeEncoder(t, false))
+		}
+	}
+
+	if t.Implements(textMarshalerType) {
+		return textMarshalerEncoder
+	}
+	if t.Kind() != reflect.Ptr && allowAddr {
+		if reflect.PtrTo(t).Implements(textMarshalerType) {
+			return newCondAddrEncoder(addrTextMarshalerEncoder, newTypeEncoder(t, false))
+		}
+	}
+
+	switch t.Kind() {
+	case reflect.Bool:
+		return boolEncoder
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		return intEncoder
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		return uintEncoder
+	case reflect.Float32:
+		return float32Encoder
+	case reflect.Float64:
+		return float64Encoder
+	case reflect.String:
+		return stringEncoder
+	case reflect.Interface:
+		return interfaceEncoder
+	case reflect.Struct:
+		return newStructEncoder(t)
+	case reflect.Map:
+		return newMapEncoder(t)
+	case reflect.Slice:
+		return newSliceEncoder(t)
+	case reflect.Array:
+		return newArrayEncoder(t)
+	case reflect.Ptr:
+		return newPtrEncoder(t)
+	default:
+		return unsupportedTypeEncoder
+	}
+}
+
+func invalidValueEncoder(e *encodeState, v reflect.Value, _ encOpts) {
+	e.WriteString("null")
+}
+
+func marshalerEncoder(e *encodeState, v reflect.Value, opts encOpts) {
+	if v.Kind() == reflect.Ptr && v.IsNil() {
+		e.WriteString("null")
+		return
+	}
+	m, ok := v.Interface().(Marshaler)
+	if !ok {
+		e.WriteString("null")
+		return
+	}
+	b, err := m.MarshalJSON()
+	if err == nil {
+		// copy JSON into buffer, checking validity.
+		err = compact(&e.Buffer, b, opts.escapeHTML)
+	}
+	if err != nil {
+		e.error(&MarshalerError{v.Type(), err})
+	}
+}
+
+func addrMarshalerEncoder(e *encodeState, v reflect.Value, _ encOpts) {
+	va := v.Addr()
+	if va.IsNil() {
+		e.WriteString("null")
+		return
+	}
+	m := va.Interface().(Marshaler)
+	b, err := m.MarshalJSON()
+	if err == nil {
+		// copy JSON into buffer, checking validity.
+		err = compact(&e.Buffer, b, true)
+	}
+	if err != nil {
+		e.error(&MarshalerError{v.Type(), err})
+	}
+}
+
+func textMarshalerEncoder(e *encodeState, v reflect.Value, opts encOpts) {
+	if v.Kind() == reflect.Ptr && v.IsNil() {
+		e.WriteString("null")
+		return
+	}
+	m := v.Interface().(encoding.TextMarshaler)
+	b, err := m.MarshalText()
+	if err != nil {
+		e.error(&MarshalerError{v.Type(), err})
+	}
+	e.stringBytes(b, opts.escapeHTML)
+}
+
+func addrTextMarshalerEncoder(e *encodeState, v reflect.Value, opts encOpts) {
+	va := v.Addr()
+	if va.IsNil() {
+		e.WriteString("null")
+		return
+	}
+	m := va.Interface().(encoding.TextMarshaler)
+	b, err := m.MarshalText()
+	if err != nil {
+		e.error(&MarshalerError{v.Type(), err})
+	}
+	e.stringBytes(b, opts.escapeHTML)
+}
+
+func boolEncoder(e *encodeState, v reflect.Value, opts encOpts) {
+	if opts.quoted {
+		e.WriteByte('"')
+	}
+	if v.Bool() {
+		e.WriteString("true")
+	} else {
+		e.WriteString("false")
+	}
+	if opts.quoted {
+		e.WriteByte('"')
+	}
+}
+
+func intEncoder(e *encodeState, v reflect.Value, opts encOpts) {
+	b := strconv.AppendInt(e.scratch[:0], v.Int(), 10)
+	if opts.quoted {
+		e.WriteByte('"')
+	}
+	e.Write(b)
+	if opts.quoted {
+		e.WriteByte('"')
+	}
+}
+
+func uintEncoder(e *encodeState, v reflect.Value, opts encOpts) {
+	b := strconv.AppendUint(e.scratch[:0], v.Uint(), 10)
+	if opts.quoted {
+		e.WriteByte('"')
+	}
+	e.Write(b)
+	if opts.quoted {
+		e.WriteByte('"')
+	}
+}
+
+type floatEncoder int // number of bits
+
+func (bits floatEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
+	f := v.Float()
+	if math.IsInf(f, 0) || math.IsNaN(f) {
+		e.error(&UnsupportedValueError{v, strconv.FormatFloat(f, 'g', -1, int(bits))})
+	}
+
+	// Convert as if by ES6 number to string conversion.
+	// This matches most other JSON generators.
+	// See golang.org/issue/6384 and golang.org/issue/14135.
+	// Like fmt %g, but the exponent cutoffs are different
+	// and exponents themselves are not padded to two digits.
+	b := e.scratch[:0]
+	abs := math.Abs(f)
+	fmt := byte('f')
+	// Note: Must use float32 comparisons for underlying float32 value to get precise cutoffs right.
+	if abs != 0 {
+		if bits == 64 && (abs < 1e-6 || abs >= 1e21) || bits == 32 && (float32(abs) < 1e-6 || float32(abs) >= 1e21) {
+			fmt = 'e'
+		}
+	}
+	b = strconv.AppendFloat(b, f, fmt, -1, int(bits))
+	if fmt == 'e' {
+		// clean up e-09 to e-9
+		n := len(b)
+		if n >= 4 && b[n-4] == 'e' && b[n-3] == '-' && b[n-2] == '0' {
+			b[n-2] = b[n-1]
+			b = b[:n-1]
+		}
+	}
+
+	if opts.quoted {
+		e.WriteByte('"')
+	}
+	e.Write(b)
+	if opts.quoted {
+		e.WriteByte('"')
+	}
+}
+
+var (
+	float32Encoder = (floatEncoder(32)).encode
+	float64Encoder = (floatEncoder(64)).encode
+)
+
+func stringEncoder(e *encodeState, v reflect.Value, opts encOpts) {
+	if v.Type() == numberType {
+		numStr := v.String()
+		// In Go1.5 the empty string encodes to "0", while this is not a valid number literal
+		// we keep compatibility so check validity after this.
+		if numStr == "" {
+			numStr = "0" // Number's zero-val
+		}
+		if !isValidNumber(numStr) {
+			e.error(fmt.Errorf("json: invalid number literal %q", numStr))
+		}
+		e.WriteString(numStr)
+		return
+	}
+	if opts.quoted {
+		sb, err := Marshal(v.String())
+		if err != nil {
+			e.error(err)
+		}
+		e.string(string(sb), opts.escapeHTML)
+	} else {
+		e.string(v.String(), opts.escapeHTML)
+	}
+}
+
+func interfaceEncoder(e *encodeState, v reflect.Value, opts encOpts) {
+	if v.IsNil() {
+		e.WriteString("null")
+		return
+	}
+	e.reflectValue(v.Elem(), opts)
+}
+
+func unsupportedTypeEncoder(e *encodeState, v reflect.Value, _ encOpts) {
+	e.error(&UnsupportedTypeError{v.Type()})
+}
+
+type structEncoder struct {
+	fields []field
+}
+
+func (se structEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
+	next := byte('{')
+FieldLoop:
+	for i := range se.fields {
+		f := &se.fields[i]
+
+		// Find the nested struct field by following f.index.
+		fv := v
+		for _, i := range f.index {
+			if fv.Kind() == reflect.Ptr {
+				if fv.IsNil() {
+					continue FieldLoop
+				}
+				fv = fv.Elem()
+			}
+			fv = fv.Field(i)
+		}
+
+		if f.omitEmpty && isEmptyValue(fv) {
+			continue
+		}
+		e.WriteByte(next)
+		next = ','
+		if opts.escapeHTML {
+			e.WriteString(f.nameEscHTML)
+		} else {
+			e.WriteString(f.nameNonEsc)
+		}
+		opts.quoted = f.quoted
+		f.encoder(e, fv, opts)
+	}
+	if next == '{' {
+		e.WriteString("{}")
+	} else {
+		e.WriteByte('}')
+	}
+}
+
+func newStructEncoder(t reflect.Type) encoderFunc {
+	se := structEncoder{fields: cachedTypeFields(t)}
+	return se.encode
+}
+
+type mapEncoder struct {
+	elemEnc encoderFunc
+}
+
+func (me mapEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
+	if v.IsNil() {
+		e.WriteString("null")
+		return
+	}
+	e.WriteByte('{')
+
+	// Extract and sort the keys.
+	keys := v.MapKeys()
+	sv := make([]reflectWithString, len(keys))
+	for i, v := range keys {
+		sv[i].v = v
+		if err := sv[i].resolve(); err != nil {
+			e.error(&MarshalerError{v.Type(), err})
+		}
+	}
+	sort.Slice(sv, func(i, j int) bool { return sv[i].s < sv[j].s })
+
+	for i, kv := range sv {
+		if i > 0 {
+			e.WriteByte(',')
+		}
+		e.string(kv.s, opts.escapeHTML)
+		e.WriteByte(':')
+		me.elemEnc(e, v.MapIndex(kv.v), opts)
+	}
+	e.WriteByte('}')
+}
+
+func newMapEncoder(t reflect.Type) encoderFunc {
+	switch t.Key().Kind() {
+	case reflect.String,
+		reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
+		reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+	default:
+		if !t.Key().Implements(textMarshalerType) {
+			return unsupportedTypeEncoder
+		}
+	}
+	me := mapEncoder{typeEncoder(t.Elem())}
+	return me.encode
+}
+
+func encodeByteSlice(e *encodeState, v reflect.Value, _ encOpts) {
+	if v.IsNil() {
+		e.WriteString("null")
+		return
+	}
+	s := v.Bytes()
+	e.WriteByte('"')
+	encodedLen := base64.StdEncoding.EncodedLen(len(s))
+	if encodedLen <= len(e.scratch) {
+		// If the encoded bytes fit in e.scratch, avoid an extra
+		// allocation and use the cheaper Encoding.Encode.
+		dst := e.scratch[:encodedLen]
+		base64.StdEncoding.Encode(dst, s)
+		e.Write(dst)
+	} else if encodedLen <= 1024 {
+		// The encoded bytes are short enough to allocate for, and
+		// Encoding.Encode is still cheaper.
+		dst := make([]byte, encodedLen)
+		base64.StdEncoding.Encode(dst, s)
+		e.Write(dst)
+	} else {
+		// The encoded bytes are too long to cheaply allocate, and
+		// Encoding.Encode is no longer noticeably cheaper.
+		enc := base64.NewEncoder(base64.StdEncoding, e)
+		enc.Write(s)
+		enc.Close()
+	}
+	e.WriteByte('"')
+}
+
+// sliceEncoder just wraps an arrayEncoder, checking to make sure the value isn't nil.
+type sliceEncoder struct {
+	arrayEnc encoderFunc
+}
+
+func (se sliceEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
+	if v.IsNil() {
+		e.WriteString("null")
+		return
+	}
+	se.arrayEnc(e, v, opts)
+}
+
+func newSliceEncoder(t reflect.Type) encoderFunc {
+	// Byte slices get special treatment; arrays don't.
+	if t.Elem().Kind() == reflect.Uint8 {
+		p := reflect.PtrTo(t.Elem())
+		if !p.Implements(marshalerType) && !p.Implements(textMarshalerType) {
+			return encodeByteSlice
+		}
+	}
+	enc := sliceEncoder{newArrayEncoder(t)}
+	return enc.encode
+}
+
+type arrayEncoder struct {
+	elemEnc encoderFunc
+}
+
+func (ae arrayEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
+	e.WriteByte('[')
+	n := v.Len()
+	for i := 0; i < n; i++ {
+		if i > 0 {
+			e.WriteByte(',')
+		}
+		ae.elemEnc(e, v.Index(i), opts)
+	}
+	e.WriteByte(']')
+}
+
+func newArrayEncoder(t reflect.Type) encoderFunc {
+	enc := arrayEncoder{typeEncoder(t.Elem())}
+	return enc.encode
+}
+
+type ptrEncoder struct {
+	elemEnc encoderFunc
+}
+
+func (pe ptrEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
+	if v.IsNil() {
+		e.WriteString("null")
+		return
+	}
+	pe.elemEnc(e, v.Elem(), opts)
+}
+
+func newPtrEncoder(t reflect.Type) encoderFunc {
+	enc := ptrEncoder{typeEncoder(t.Elem())}
+	return enc.encode
+}
+
+type condAddrEncoder struct {
+	canAddrEnc, elseEnc encoderFunc
+}
+
+func (ce condAddrEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
+	if v.CanAddr() {
+		ce.canAddrEnc(e, v, opts)
+	} else {
+		ce.elseEnc(e, v, opts)
+	}
+}
+
+// newCondAddrEncoder returns an encoder that checks whether its value
+// CanAddr and delegates to canAddrEnc if so, else to elseEnc.
+func newCondAddrEncoder(canAddrEnc, elseEnc encoderFunc) encoderFunc {
+	enc := condAddrEncoder{canAddrEnc: canAddrEnc, elseEnc: elseEnc}
+	return enc.encode
+}
+
+func isValidTag(s string) bool {
+	if s == "" {
+		return false
+	}
+	for _, c := range s {
+		switch {
+		case strings.ContainsRune("!#$%&()*+-./:<=>?@[]^_{|}~ ", c):
+			// Backslash and quote chars are reserved, but
+			// otherwise any punctuation chars are allowed
+			// in a tag name.
+		case !unicode.IsLetter(c) && !unicode.IsDigit(c):
+			return false
+		}
+	}
+	return true
+}
+
+func typeByIndex(t reflect.Type, index []int) reflect.Type {
+	for _, i := range index {
+		if t.Kind() == reflect.Ptr {
+			t = t.Elem()
+		}
+		t = t.Field(i).Type
+	}
+	return t
+}
+
+type reflectWithString struct {
+	v reflect.Value
+	s string
+}
+
+func (w *reflectWithString) resolve() error {
+	if w.v.Kind() == reflect.String {
+		w.s = w.v.String()
+		return nil
+	}
+	if tm, ok := w.v.Interface().(encoding.TextMarshaler); ok {
+		buf, err := tm.MarshalText()
+		w.s = string(buf)
+		return err
+	}
+	switch w.v.Kind() {
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		w.s = strconv.FormatInt(w.v.Int(), 10)
+		return nil
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		w.s = strconv.FormatUint(w.v.Uint(), 10)
+		return nil
+	}
+	panic("unexpected map key type")
+}
+
+// NOTE: keep in sync with stringBytes below.
+func (e *encodeState) string(s string, escapeHTML bool) {
+	e.WriteByte('"')
+	start := 0
+	for i := 0; i < len(s); {
+		if b := s[i]; b < utf8.RuneSelf {
+			if htmlSafeSet[b] || (!escapeHTML && safeSet[b]) {
+				i++
+				continue
+			}
+			if start < i {
+				e.WriteString(s[start:i])
+			}
+			e.WriteByte('\\')
+			switch b {
+			case '\\', '"':
+				e.WriteByte(b)
+			case '\n':
+				e.WriteByte('n')
+			case '\r':
+				e.WriteByte('r')
+			case '\t':
+				e.WriteByte('t')
+			default:
+				// This encodes bytes < 0x20 except for \t, \n and \r.
+				// If escapeHTML is set, it also escapes <, >, and &
+				// because they can lead to security holes when
+				// user-controlled strings are rendered into JSON
+				// and served to some browsers.
+				e.WriteString(`u00`)
+				e.WriteByte(hex[b>>4])
+				e.WriteByte(hex[b&0xF])
+			}
+			i++
+			start = i
+			continue
+		}
+		c, size := utf8.DecodeRuneInString(s[i:])
+		if c == utf8.RuneError && size == 1 {
+			if start < i {
+				e.WriteString(s[start:i])
+			}
+			e.WriteString(`\ufffd`)
+			i += size
+			start = i
+			continue
+		}
+		// U+2028 is LINE SEPARATOR.
+		// U+2029 is PARAGRAPH SEPARATOR.
+		// They are both technically valid characters in JSON strings,
+		// but don't work in JSONP, which has to be evaluated as JavaScript,
+		// and can lead to security holes there. It is valid JSON to
+		// escape them, so we do so unconditionally.
+		// See http://timelessrepo.com/json-isnt-a-javascript-subset for discussion.
+		if c == '\u2028' || c == '\u2029' {
+			if start < i {
+				e.WriteString(s[start:i])
+			}
+			e.WriteString(`\u202`)
+			e.WriteByte(hex[c&0xF])
+			i += size
+			start = i
+			continue
+		}
+		i += size
+	}
+	if start < len(s) {
+		e.WriteString(s[start:])
+	}
+	e.WriteByte('"')
+}
+
+// NOTE: keep in sync with string above.
+func (e *encodeState) stringBytes(s []byte, escapeHTML bool) {
+	e.WriteByte('"')
+	start := 0
+	for i := 0; i < len(s); {
+		if b := s[i]; b < utf8.RuneSelf {
+			if htmlSafeSet[b] || (!escapeHTML && safeSet[b]) {
+				i++
+				continue
+			}
+			if start < i {
+				e.Write(s[start:i])
+			}
+			e.WriteByte('\\')
+			switch b {
+			case '\\', '"':
+				e.WriteByte(b)
+			case '\n':
+				e.WriteByte('n')
+			case '\r':
+				e.WriteByte('r')
+			case '\t':
+				e.WriteByte('t')
+			default:
+				// This encodes bytes < 0x20 except for \t, \n and \r.
+				// If escapeHTML is set, it also escapes <, >, and &
+				// because they can lead to security holes when
+				// user-controlled strings are rendered into JSON
+				// and served to some browsers.
+				e.WriteString(`u00`)
+				e.WriteByte(hex[b>>4])
+				e.WriteByte(hex[b&0xF])
+			}
+			i++
+			start = i
+			continue
+		}
+		c, size := utf8.DecodeRune(s[i:])
+		if c == utf8.RuneError && size == 1 {
+			if start < i {
+				e.Write(s[start:i])
+			}
+			e.WriteString(`\ufffd`)
+			i += size
+			start = i
+			continue
+		}
+		// U+2028 is LINE SEPARATOR.
+		// U+2029 is PARAGRAPH SEPARATOR.
+		// They are both technically valid characters in JSON strings,
+		// but don't work in JSONP, which has to be evaluated as JavaScript,
+		// and can lead to security holes there. It is valid JSON to
+		// escape them, so we do so unconditionally.
+		// See http://timelessrepo.com/json-isnt-a-javascript-subset for discussion.
+		if c == '\u2028' || c == '\u2029' {
+			if start < i {
+				e.Write(s[start:i])
+			}
+			e.WriteString(`\u202`)
+			e.WriteByte(hex[c&0xF])
+			i += size
+			start = i
+			continue
+		}
+		i += size
+	}
+	if start < len(s) {
+		e.Write(s[start:])
+	}
+	e.WriteByte('"')
+}
+
+// A field represents a single field found in a struct.
+type field struct {
+	name      string
+	nameBytes []byte                 // []byte(name)
+	equalFold func(s, t []byte) bool // bytes.EqualFold or equivalent
+
+	nameNonEsc  string // `"` + name + `":`
+	nameEscHTML string // `"` + HTMLEscape(name) + `":`
+
+	tag       bool
+	index     []int
+	typ       reflect.Type
+	omitEmpty bool
+	quoted    bool
+
+	encoder encoderFunc
+}
+
+// byIndex sorts field by index sequence.
+type byIndex []field
+
+func (x byIndex) Len() int { return len(x) }
+
+func (x byIndex) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
+
+func (x byIndex) Less(i, j int) bool {
+	for k, xik := range x[i].index {
+		if k >= len(x[j].index) {
+			return false
+		}
+		if xik != x[j].index[k] {
+			return xik < x[j].index[k]
+		}
+	}
+	return len(x[i].index) < len(x[j].index)
+}
+
+// typeFields returns a list of fields that JSON should recognize for the given type.
+// The algorithm is breadth-first search over the set of structs to include - the top struct
+// and then any reachable anonymous structs.
+func typeFields(t reflect.Type) []field {
+	// Anonymous fields to explore at the current level and the next.
+	current := []field{}
+	next := []field{{typ: t}}
+
+	// Count of queued names for current level and the next.
+	count := map[reflect.Type]int{}
+	nextCount := map[reflect.Type]int{}
+
+	// Types already visited at an earlier level.
+	visited := map[reflect.Type]bool{}
+
+	// Fields found.
+	var fields []field
+
+	// Buffer to run HTMLEscape on field names.
+	var nameEscBuf bytes.Buffer
+
+	for len(next) > 0 {
+		current, next = next, current[:0]
+		count, nextCount = nextCount, map[reflect.Type]int{}
+
+		for _, f := range current {
+			if visited[f.typ] {
+				continue
+			}
+			visited[f.typ] = true
+
+			// Scan f.typ for fields to include.
+			for i := 0; i < f.typ.NumField(); i++ {
+				sf := f.typ.Field(i)
+				isUnexported := sf.PkgPath != ""
+				if sf.Anonymous {
+					t := sf.Type
+					if t.Kind() == reflect.Ptr {
+						t = t.Elem()
+					}
+					if isUnexported && t.Kind() != reflect.Struct {
+						// Ignore embedded fields of unexported non-struct types.
+						continue
+					}
+					// Do not ignore embedded fields of unexported struct types
+					// since they may have exported fields.
+				} else if isUnexported {
+					// Ignore unexported non-embedded fields.
+					continue
+				}
+				tag := sf.Tag.Get("json")
+				if tag == "-" {
+					continue
+				}
+				name, opts := parseTag(tag)
+				if !isValidTag(name) {
+					name = ""
+				}
+				index := make([]int, len(f.index)+1)
+				copy(index, f.index)
+				index[len(f.index)] = i
+
+				ft := sf.Type
+				if ft.Name() == "" && ft.Kind() == reflect.Ptr {
+					// Follow pointer.
+					ft = ft.Elem()
+				}
+
+				// Only strings, floats, integers, and booleans can be quoted.
+				quoted := false
+				if opts.Contains("string") {
+					switch ft.Kind() {
+					case reflect.Bool,
+						reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
+						reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr,
+						reflect.Float32, reflect.Float64,
+						reflect.String:
+						quoted = true
+					}
+				}
+
+				// Record found field and index sequence.
+				if name != "" || !sf.Anonymous || ft.Kind() != reflect.Struct {
+					tagged := name != ""
+					if name == "" {
+						name = sf.Name
+					}
+					field := field{
+						name:      name,
+						tag:       tagged,
+						index:     index,
+						typ:       ft,
+						omitEmpty: opts.Contains("omitempty"),
+						quoted:    quoted,
+					}
+					field.nameBytes = []byte(field.name)
+					field.equalFold = foldFunc(field.nameBytes)
+
+					// Build nameEscHTML and nameNonEsc ahead of time.
+					nameEscBuf.Reset()
+					nameEscBuf.WriteString(`"`)
+					HTMLEscape(&nameEscBuf, field.nameBytes)
+					nameEscBuf.WriteString(`":`)
+					field.nameEscHTML = nameEscBuf.String()
+					field.nameNonEsc = `"` + field.name + `":`
+
+					fields = append(fields, field)
+					if count[f.typ] > 1 {
+						// If there were multiple instances, add a second,
+						// so that the annihilation code will see a duplicate.
+						// It only cares about the distinction between 1 or 2,
+						// so don't bother generating any more copies.
+						fields = append(fields, fields[len(fields)-1])
+					}
+					continue
+				}
+
+				// Record new anonymous struct to explore in next round.
+				nextCount[ft]++
+				if nextCount[ft] == 1 {
+					next = append(next, field{name: ft.Name(), index: index, typ: ft})
+				}
+			}
+		}
+	}
+
+	sort.Slice(fields, func(i, j int) bool {
+		x := fields
+		// sort field by name, breaking ties with depth, then
+		// breaking ties with "name came from json tag", then
+		// breaking ties with index sequence.
+		if x[i].name != x[j].name {
+			return x[i].name < x[j].name
+		}
+		if len(x[i].index) != len(x[j].index) {
+			return len(x[i].index) < len(x[j].index)
+		}
+		if x[i].tag != x[j].tag {
+			return x[i].tag
+		}
+		return byIndex(x).Less(i, j)
+	})
+
+	// Delete all fields that are hidden by the Go rules for embedded fields,
+	// except that fields with JSON tags are promoted.
+
+	// The fields are sorted in primary order of name, secondary order
+	// of field index length. Loop over names; for each name, delete
+	// hidden fields by choosing the one dominant field that survives.
+	out := fields[:0]
+	for advance, i := 0, 0; i < len(fields); i += advance {
+		// One iteration per name.
+		// Find the sequence of fields with the name of this first field.
+		fi := fields[i]
+		name := fi.name
+		for advance = 1; i+advance < len(fields); advance++ {
+			fj := fields[i+advance]
+			if fj.name != name {
+				break
+			}
+		}
+		if advance == 1 { // Only one field with this name
+			out = append(out, fi)
+			continue
+		}
+		dominant, ok := dominantField(fields[i : i+advance])
+		if ok {
+			out = append(out, dominant)
+		}
+	}
+
+	fields = out
+	sort.Sort(byIndex(fields))
+
+	for i := range fields {
+		f := &fields[i]
+		f.encoder = typeEncoder(typeByIndex(t, f.index))
+	}
+	return fields
+}
+
+// dominantField looks through the fields, all of which are known to
+// have the same name, to find the single field that dominates the
+// others using Go's embedding rules, modified by the presence of
+// JSON tags. If there are multiple top-level fields, the boolean
+// will be false: This condition is an error in Go and we skip all
+// the fields.
+func dominantField(fields []field) (field, bool) {
+	// The fields are sorted in increasing index-length order, then by presence of tag.
+	// That means that the first field is the dominant one. We need only check
+	// for error cases: two fields at top level, either both tagged or neither tagged.
+	if len(fields) > 1 && len(fields[0].index) == len(fields[1].index) && fields[0].tag == fields[1].tag {
+		return field{}, false
+	}
+	return fields[0], true
+}
+
+var fieldCache sync.Map // map[reflect.Type][]field
+
+// cachedTypeFields is like typeFields but uses a cache to avoid repeated work.
+func cachedTypeFields(t reflect.Type) []field {
+	if f, ok := fieldCache.Load(t); ok {
+		return f.([]field)
+	}
+	f, _ := fieldCache.LoadOrStore(t, typeFields(t))
+	return f.([]field)
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/encode_test.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/encode_test.go
new file mode 100644
index 0000000..cd5eadf
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/encode_test.go
@@ -0,0 +1,1025 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package json
+
+import (
+	"bytes"
+	"fmt"
+	"log"
+	"math"
+	"reflect"
+	"regexp"
+	"strconv"
+	"testing"
+	"unicode"
+)
+
+type Optionals struct {
+	Sr string `json:"sr"`
+	So string `json:"so,omitempty"`
+	Sw string `json:"-"`
+
+	Ir int `json:"omitempty"` // actually named omitempty, not an option
+	Io int `json:"io,omitempty"`
+
+	Slr []string `json:"slr,random"`
+	Slo []string `json:"slo,omitempty"`
+
+	Mr map[string]interface{} `json:"mr"`
+	Mo map[string]interface{} `json:",omitempty"`
+
+	Fr float64 `json:"fr"`
+	Fo float64 `json:"fo,omitempty"`
+
+	Br bool `json:"br"`
+	Bo bool `json:"bo,omitempty"`
+
+	Ur uint `json:"ur"`
+	Uo uint `json:"uo,omitempty"`
+
+	Str struct{} `json:"str"`
+	Sto struct{} `json:"sto,omitempty"`
+}
+
+var optionalsExpected = `{
+ "sr": "",
+ "omitempty": 0,
+ "slr": null,
+ "mr": {},
+ "fr": 0,
+ "br": false,
+ "ur": 0,
+ "str": {},
+ "sto": {}
+}`
+
+func TestOmitEmpty(t *testing.T) {
+	var o Optionals
+	o.Sw = "something"
+	o.Mr = map[string]interface{}{}
+	o.Mo = map[string]interface{}{}
+
+	got, err := MarshalIndent(&o, "", " ")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := string(got); got != optionalsExpected {
+		t.Errorf(" got: %s\nwant: %s\n", got, optionalsExpected)
+	}
+}
+
+type StringTag struct {
+	BoolStr    bool    `json:",string"`
+	IntStr     int64   `json:",string"`
+	UintptrStr uintptr `json:",string"`
+	StrStr     string  `json:",string"`
+}
+
+var stringTagExpected = `{
+ "BoolStr": "true",
+ "IntStr": "42",
+ "UintptrStr": "44",
+ "StrStr": "\"xzbit\""
+}`
+
+func TestStringTag(t *testing.T) {
+	var s StringTag
+	s.BoolStr = true
+	s.IntStr = 42
+	s.UintptrStr = 44
+	s.StrStr = "xzbit"
+	got, err := MarshalIndent(&s, "", " ")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := string(got); got != stringTagExpected {
+		t.Fatalf(" got: %s\nwant: %s\n", got, stringTagExpected)
+	}
+
+	// Verify that it round-trips.
+	var s2 StringTag
+	err = NewDecoder(bytes.NewReader(got)).Decode(&s2)
+	if err != nil {
+		t.Fatalf("Decode: %v", err)
+	}
+	if !reflect.DeepEqual(s, s2) {
+		t.Fatalf("decode didn't match.\nsource: %#v\nEncoded as:\n%s\ndecode: %#v", s, string(got), s2)
+	}
+}
+
+// byte slices are special even if they're renamed types.
+type renamedByte byte
+type renamedByteSlice []byte
+type renamedRenamedByteSlice []renamedByte
+
+func TestEncodeRenamedByteSlice(t *testing.T) {
+	s := renamedByteSlice("abc")
+	result, err := Marshal(s)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expect := `"YWJj"`
+	if string(result) != expect {
+		t.Errorf(" got %s want %s", result, expect)
+	}
+	r := renamedRenamedByteSlice("abc")
+	result, err = Marshal(r)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(result) != expect {
+		t.Errorf(" got %s want %s", result, expect)
+	}
+}
+
+var unsupportedValues = []interface{}{
+	math.NaN(),
+	math.Inf(-1),
+	math.Inf(1),
+}
+
+func TestUnsupportedValues(t *testing.T) {
+	for _, v := range unsupportedValues {
+		if _, err := Marshal(v); err != nil {
+			if _, ok := err.(*UnsupportedValueError); !ok {
+				t.Errorf("for %v, got %T want UnsupportedValueError", v, err)
+			}
+		} else {
+			t.Errorf("for %v, expected error", v)
+		}
+	}
+}
+
+// Ref has Marshaler and Unmarshaler methods with pointer receiver.
+type Ref int
+
+func (*Ref) MarshalJSON() ([]byte, error) {
+	return []byte(`"ref"`), nil
+}
+
+func (r *Ref) UnmarshalJSON([]byte) error {
+	*r = 12
+	return nil
+}
+
+// Val has Marshaler methods with value receiver.
+type Val int
+
+func (Val) MarshalJSON() ([]byte, error) {
+	return []byte(`"val"`), nil
+}
+
+// RefText has Marshaler and Unmarshaler methods with pointer receiver.
+type RefText int
+
+func (*RefText) MarshalText() ([]byte, error) {
+	return []byte(`"ref"`), nil
+}
+
+func (r *RefText) UnmarshalText([]byte) error {
+	*r = 13
+	return nil
+}
+
+// ValText has Marshaler methods with value receiver.
+type ValText int
+
+func (ValText) MarshalText() ([]byte, error) {
+	return []byte(`"val"`), nil
+}
+
+func TestRefValMarshal(t *testing.T) {
+	var s = struct {
+		R0 Ref
+		R1 *Ref
+		R2 RefText
+		R3 *RefText
+		V0 Val
+		V1 *Val
+		V2 ValText
+		V3 *ValText
+	}{
+		R0: 12,
+		R1: new(Ref),
+		R2: 14,
+		R3: new(RefText),
+		V0: 13,
+		V1: new(Val),
+		V2: 15,
+		V3: new(ValText),
+	}
+	const want = `{"R0":"ref","R1":"ref","R2":"\"ref\"","R3":"\"ref\"","V0":"val","V1":"val","V2":"\"val\"","V3":"\"val\""}`
+	b, err := Marshal(&s)
+	if err != nil {
+		t.Fatalf("Marshal: %v", err)
+	}
+	if got := string(b); got != want {
+		t.Errorf("got %q, want %q", got, want)
+	}
+}
+
+// C implements Marshaler and returns unescaped JSON.
+type C int
+
+func (C) MarshalJSON() ([]byte, error) {
+	return []byte(`"<&>"`), nil
+}
+
+// CText implements Marshaler and returns unescaped text.
+type CText int
+
+func (CText) MarshalText() ([]byte, error) {
+	return []byte(`"<&>"`), nil
+}
+
+func TestMarshalerEscaping(t *testing.T) {
+	var c C
+	want := `"\u003c\u0026\u003e"`
+	b, err := Marshal(c)
+	if err != nil {
+		t.Fatalf("Marshal(c): %v", err)
+	}
+	if got := string(b); got != want {
+		t.Errorf("Marshal(c) = %#q, want %#q", got, want)
+	}
+
+	var ct CText
+	want = `"\"\u003c\u0026\u003e\""`
+	b, err = Marshal(ct)
+	if err != nil {
+		t.Fatalf("Marshal(ct): %v", err)
+	}
+	if got := string(b); got != want {
+		t.Errorf("Marshal(ct) = %#q, want %#q", got, want)
+	}
+}
+
+func TestAnonymousFields(t *testing.T) {
+	tests := []struct {
+		label     string             // Test name
+		makeInput func() interface{} // Function to create input value
+		want      string             // Expected JSON output
+	}{{
+		// Both S1 and S2 have a field named X. From the perspective of S,
+		// it is ambiguous which one X refers to.
+		// This should not serialize either field.
+		label: "AmbiguousField",
+		makeInput: func() interface{} {
+			type (
+				S1 struct{ x, X int }
+				S2 struct{ x, X int }
+				S  struct {
+					S1
+					S2
+				}
+			)
+			return S{S1{1, 2}, S2{3, 4}}
+		},
+		want: `{}`,
+	}, {
+		label: "DominantField",
+		// Both S1 and S2 have a field named X, but since S has an X field as
+		// well, it takes precedence over S1.X and S2.X.
+		makeInput: func() interface{} {
+			type (
+				S1 struct{ x, X int }
+				S2 struct{ x, X int }
+				S  struct {
+					S1
+					S2
+					x, X int
+				}
+			)
+			return S{S1{1, 2}, S2{3, 4}, 5, 6}
+		},
+		want: `{"X":6}`,
+	}, {
+		// Unexported embedded field of non-struct type should not be serialized.
+		label: "UnexportedEmbeddedInt",
+		makeInput: func() interface{} {
+			type (
+				myInt int
+				S     struct{ myInt }
+			)
+			return S{5}
+		},
+		want: `{}`,
+	}, {
+		// Exported embedded field of non-struct type should be serialized.
+		label: "ExportedEmbeddedInt",
+		makeInput: func() interface{} {
+			type (
+				MyInt int
+				S     struct{ MyInt }
+			)
+			return S{5}
+		},
+		want: `{"MyInt":5}`,
+	}, {
+		// Unexported embedded field of pointer to non-struct type
+		// should not be serialized.
+		label: "UnexportedEmbeddedIntPointer",
+		makeInput: func() interface{} {
+			type (
+				myInt int
+				S     struct{ *myInt }
+			)
+			s := S{new(myInt)}
+			*s.myInt = 5
+			return s
+		},
+		want: `{}`,
+	}, {
+		// Exported embedded field of pointer to non-struct type
+		// should be serialized.
+		label: "ExportedEmbeddedIntPointer",
+		makeInput: func() interface{} {
+			type (
+				MyInt int
+				S     struct{ *MyInt }
+			)
+			s := S{new(MyInt)}
+			*s.MyInt = 5
+			return s
+		},
+		want: `{"MyInt":5}`,
+	}, {
+		// Exported fields of embedded structs should have their
+		// exported fields be serialized regardless of whether the struct types
+		// themselves are exported.
+		label: "EmbeddedStruct",
+		makeInput: func() interface{} {
+			type (
+				s1 struct{ x, X int }
+				S2 struct{ y, Y int }
+				S  struct {
+					s1
+					S2
+				}
+			)
+			return S{s1{1, 2}, S2{3, 4}}
+		},
+		want: `{"X":2,"Y":4}`,
+	}, {
+		// Exported fields of pointers to embedded structs should have their
+		// exported fields be serialized regardless of whether the struct types
+		// themselves are exported.
+		label: "EmbeddedStructPointer",
+		makeInput: func() interface{} {
+			type (
+				s1 struct{ x, X int }
+				S2 struct{ y, Y int }
+				S  struct {
+					*s1
+					*S2
+				}
+			)
+			return S{&s1{1, 2}, &S2{3, 4}}
+		},
+		want: `{"X":2,"Y":4}`,
+	}, {
+		// Exported fields on embedded unexported structs at multiple levels
+		// of nesting should still be serialized.
+		label: "NestedStructAndInts",
+		makeInput: func() interface{} {
+			type (
+				MyInt1 int
+				MyInt2 int
+				myInt  int
+				s2     struct {
+					MyInt2
+					myInt
+				}
+				s1 struct {
+					MyInt1
+					myInt
+					s2
+				}
+				S struct {
+					s1
+					myInt
+				}
+			)
+			return S{s1{1, 2, s2{3, 4}}, 6}
+		},
+		want: `{"MyInt1":1,"MyInt2":3}`,
+	}, {
+		// If an anonymous struct pointer field is nil, we should ignore
+		// the embedded fields behind it. Not properly doing so may
+		// result in the wrong output or reflect panics.
+		label: "EmbeddedFieldBehindNilPointer",
+		makeInput: func() interface{} {
+			type (
+				S2 struct{ Field string }
+				S  struct{ *S2 }
+			)
+			return S{}
+		},
+		want: `{}`,
+	}}
+
+	for _, tt := range tests {
+		t.Run(tt.label, func(t *testing.T) {
+			b, err := Marshal(tt.makeInput())
+			if err != nil {
+				t.Fatalf("Marshal() = %v, want nil error", err)
+			}
+			if string(b) != tt.want {
+				t.Fatalf("Marshal() = %q, want %q", b, tt.want)
+			}
+		})
+	}
+}
+
+type BugA struct {
+	S string
+}
+
+type BugB struct {
+	BugA
+	S string
+}
+
+type BugC struct {
+	S string
+}
+
+// Legal Go: We never use the repeated embedded field (S).
+type BugX struct {
+	A int
+	BugA
+	BugB
+}
+
+// Issue 16042. Even if a nil interface value is passed in
+// as long as it implements MarshalJSON, it should be marshaled.
+type nilMarshaler string
+
+func (nm *nilMarshaler) MarshalJSON() ([]byte, error) {
+	if nm == nil {
+		return Marshal("0zenil0")
+	}
+	return Marshal("zenil:" + string(*nm))
+}
+
+// Issue 16042.
+func TestNilMarshal(t *testing.T) {
+	testCases := []struct {
+		v    interface{}
+		want string
+	}{
+		{v: nil, want: `null`},
+		{v: new(float64), want: `0`},
+		{v: []interface{}(nil), want: `null`},
+		{v: []string(nil), want: `null`},
+		{v: map[string]string(nil), want: `null`},
+		{v: []byte(nil), want: `null`},
+		{v: struct{ M string }{"gopher"}, want: `{"M":"gopher"}`},
+		{v: struct{ M Marshaler }{}, want: `{"M":null}`},
+		{v: struct{ M Marshaler }{(*nilMarshaler)(nil)}, want: `{"M":"0zenil0"}`},
+		{v: struct{ M interface{} }{(*nilMarshaler)(nil)}, want: `{"M":null}`},
+	}
+
+	for _, tt := range testCases {
+		out, err := Marshal(tt.v)
+		if err != nil || string(out) != tt.want {
+			t.Errorf("Marshal(%#v) = %#q, %#v, want %#q, nil", tt.v, out, err, tt.want)
+			continue
+		}
+	}
+}
+
+// Issue 5245.
+func TestEmbeddedBug(t *testing.T) {
+	v := BugB{
+		BugA{"A"},
+		"B",
+	}
+	b, err := Marshal(v)
+	if err != nil {
+		t.Fatal("Marshal:", err)
+	}
+	want := `{"S":"B"}`
+	got := string(b)
+	if got != want {
+		t.Fatalf("Marshal: got %s want %s", got, want)
+	}
+	// Now check that the duplicate field, S, does not appear.
+	x := BugX{
+		A: 23,
+	}
+	b, err = Marshal(x)
+	if err != nil {
+		t.Fatal("Marshal:", err)
+	}
+	want = `{"A":23}`
+	got = string(b)
+	if got != want {
+		t.Fatalf("Marshal: got %s want %s", got, want)
+	}
+}
+
+type BugD struct { // Same as BugA after tagging.
+	XXX string `json:"S"`
+}
+
+// BugD's tagged S field should dominate BugA's.
+type BugY struct {
+	BugA
+	BugD
+}
+
+// Test that a field with a tag dominates untagged fields.
+func TestTaggedFieldDominates(t *testing.T) {
+	v := BugY{
+		BugA{"BugA"},
+		BugD{"BugD"},
+	}
+	b, err := Marshal(v)
+	if err != nil {
+		t.Fatal("Marshal:", err)
+	}
+	want := `{"S":"BugD"}`
+	got := string(b)
+	if got != want {
+		t.Fatalf("Marshal: got %s want %s", got, want)
+	}
+}
+
+// There are no tags here, so S should not appear.
+type BugZ struct {
+	BugA
+	BugC
+	BugY // Contains a tagged S field through BugD; should not dominate.
+}
+
+func TestDuplicatedFieldDisappears(t *testing.T) {
+	v := BugZ{
+		BugA{"BugA"},
+		BugC{"BugC"},
+		BugY{
+			BugA{"nested BugA"},
+			BugD{"nested BugD"},
+		},
+	}
+	b, err := Marshal(v)
+	if err != nil {
+		t.Fatal("Marshal:", err)
+	}
+	want := `{}`
+	got := string(b)
+	if got != want {
+		t.Fatalf("Marshal: got %s want %s", got, want)
+	}
+}
+
+func TestStringBytes(t *testing.T) {
+	t.Parallel()
+	// Test that encodeState.stringBytes and encodeState.string use the same encoding.
+	var r []rune
+	for i := '\u0000'; i <= unicode.MaxRune; i++ {
+		r = append(r, i)
+	}
+	s := string(r) + "\xff\xff\xffhello" // some invalid UTF-8 too
+
+	for _, escapeHTML := range []bool{true, false} {
+		es := &encodeState{}
+		es.string(s, escapeHTML)
+
+		esBytes := &encodeState{}
+		esBytes.stringBytes([]byte(s), escapeHTML)
+
+		enc := es.Buffer.String()
+		encBytes := esBytes.Buffer.String()
+		if enc != encBytes {
+			i := 0
+			for i < len(enc) && i < len(encBytes) && enc[i] == encBytes[i] {
+				i++
+			}
+			enc = enc[i:]
+			encBytes = encBytes[i:]
+			i = 0
+			for i < len(enc) && i < len(encBytes) && enc[len(enc)-i-1] == encBytes[len(encBytes)-i-1] {
+				i++
+			}
+			enc = enc[:len(enc)-i]
+			encBytes = encBytes[:len(encBytes)-i]
+
+			if len(enc) > 20 {
+				enc = enc[:20] + "..."
+			}
+			if len(encBytes) > 20 {
+				encBytes = encBytes[:20] + "..."
+			}
+
+			t.Errorf("with escapeHTML=%t, encodings differ at %#q vs %#q",
+				escapeHTML, enc, encBytes)
+		}
+	}
+}
+
+func TestIssue10281(t *testing.T) {
+	type Foo struct {
+		N Number
+	}
+	x := Foo{Number(`invalid`)}
+
+	b, err := Marshal(&x)
+	if err == nil {
+		t.Errorf("Marshal(&x) = %#q; want error", b)
+	}
+}
+
+func TestHTMLEscape(t *testing.T) {
+	var b, want bytes.Buffer
+	m := `{"M":"<html>foo &` + "\xe2\x80\xa8 \xe2\x80\xa9" + `</html>"}`
+	want.Write([]byte(`{"M":"\u003chtml\u003efoo \u0026\u2028 \u2029\u003c/html\u003e"}`))
+	HTMLEscape(&b, []byte(m))
+	if !bytes.Equal(b.Bytes(), want.Bytes()) {
+		t.Errorf("HTMLEscape(&b, []byte(m)) = %s; want %s", b.Bytes(), want.Bytes())
+	}
+}
+
+// golang.org/issue/8582
+func TestEncodePointerString(t *testing.T) {
+	type stringPointer struct {
+		N *int64 `json:"n,string"`
+	}
+	var n int64 = 42
+	b, err := Marshal(stringPointer{N: &n})
+	if err != nil {
+		t.Fatalf("Marshal: %v", err)
+	}
+	if got, want := string(b), `{"n":"42"}`; got != want {
+		t.Errorf("Marshal = %s, want %s", got, want)
+	}
+	var back stringPointer
+	err = Unmarshal(b, &back)
+	if err != nil {
+		t.Fatalf("Unmarshal: %v", err)
+	}
+	if back.N == nil {
+		t.Fatalf("Unmarshaled nil N field")
+	}
+	if *back.N != 42 {
+		t.Fatalf("*N = %d; want 42", *back.N)
+	}
+}
+
+var encodeStringTests = []struct {
+	in  string
+	out string
+}{
+	{"\x00", `"\u0000"`},
+	{"\x01", `"\u0001"`},
+	{"\x02", `"\u0002"`},
+	{"\x03", `"\u0003"`},
+	{"\x04", `"\u0004"`},
+	{"\x05", `"\u0005"`},
+	{"\x06", `"\u0006"`},
+	{"\x07", `"\u0007"`},
+	{"\x08", `"\u0008"`},
+	{"\x09", `"\t"`},
+	{"\x0a", `"\n"`},
+	{"\x0b", `"\u000b"`},
+	{"\x0c", `"\u000c"`},
+	{"\x0d", `"\r"`},
+	{"\x0e", `"\u000e"`},
+	{"\x0f", `"\u000f"`},
+	{"\x10", `"\u0010"`},
+	{"\x11", `"\u0011"`},
+	{"\x12", `"\u0012"`},
+	{"\x13", `"\u0013"`},
+	{"\x14", `"\u0014"`},
+	{"\x15", `"\u0015"`},
+	{"\x16", `"\u0016"`},
+	{"\x17", `"\u0017"`},
+	{"\x18", `"\u0018"`},
+	{"\x19", `"\u0019"`},
+	{"\x1a", `"\u001a"`},
+	{"\x1b", `"\u001b"`},
+	{"\x1c", `"\u001c"`},
+	{"\x1d", `"\u001d"`},
+	{"\x1e", `"\u001e"`},
+	{"\x1f", `"\u001f"`},
+}
+
+func TestEncodeString(t *testing.T) {
+	for _, tt := range encodeStringTests {
+		b, err := Marshal(tt.in)
+		if err != nil {
+			t.Errorf("Marshal(%q): %v", tt.in, err)
+			continue
+		}
+		out := string(b)
+		if out != tt.out {
+			t.Errorf("Marshal(%q) = %#q, want %#q", tt.in, out, tt.out)
+		}
+	}
+}
+
+type jsonbyte byte
+
+func (b jsonbyte) MarshalJSON() ([]byte, error) { return tenc(`{"JB":%d}`, b) }
+
+type textbyte byte
+
+func (b textbyte) MarshalText() ([]byte, error) { return tenc(`TB:%d`, b) }
+
+type jsonint int
+
+func (i jsonint) MarshalJSON() ([]byte, error) { return tenc(`{"JI":%d}`, i) }
+
+type textint int
+
+func (i textint) MarshalText() ([]byte, error) { return tenc(`TI:%d`, i) }
+
+func tenc(format string, a ...interface{}) ([]byte, error) {
+	var buf bytes.Buffer
+	fmt.Fprintf(&buf, format, a...)
+	return buf.Bytes(), nil
+}
+
+// Issue 13783
+func TestEncodeBytekind(t *testing.T) {
+	testdata := []struct {
+		data interface{}
+		want string
+	}{
+		{byte(7), "7"},
+		{jsonbyte(7), `{"JB":7}`},
+		{textbyte(4), `"TB:4"`},
+		{jsonint(5), `{"JI":5}`},
+		{textint(1), `"TI:1"`},
+		{[]byte{0, 1}, `"AAE="`},
+		{[]jsonbyte{0, 1}, `[{"JB":0},{"JB":1}]`},
+		{[][]jsonbyte{{0, 1}, {3}}, `[[{"JB":0},{"JB":1}],[{"JB":3}]]`},
+		{[]textbyte{2, 3}, `["TB:2","TB:3"]`},
+		{[]jsonint{5, 4}, `[{"JI":5},{"JI":4}]`},
+		{[]textint{9, 3}, `["TI:9","TI:3"]`},
+		{[]int{9, 3}, `[9,3]`},
+	}
+	for _, d := range testdata {
+		js, err := Marshal(d.data)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+		got, want := string(js), d.want
+		if got != want {
+			t.Errorf("got %s, want %s", got, want)
+		}
+	}
+}
+
+func TestTextMarshalerMapKeysAreSorted(t *testing.T) {
+	b, err := Marshal(map[unmarshalerText]int{
+		{"x", "y"}: 1,
+		{"y", "x"}: 2,
+		{"a", "z"}: 3,
+		{"z", "a"}: 4,
+	})
+	if err != nil {
+		t.Fatalf("Failed to Marshal text.Marshaler: %v", err)
+	}
+	const want = `{"a:z":3,"x:y":1,"y:x":2,"z:a":4}`
+	if string(b) != want {
+		t.Errorf("Marshal map with text.Marshaler keys: got %#q, want %#q", b, want)
+	}
+}
+
+var re = regexp.MustCompile
+
+// syntactic checks on form of marshaled floating point numbers.
+var badFloatREs = []*regexp.Regexp{
+	re(`p`),                     // no binary exponential notation
+	re(`^\+`),                   // no leading + sign
+	re(`^-?0[^.]`),              // no unnecessary leading zeros
+	re(`^-?\.`),                 // leading zero required before decimal point
+	re(`\.(e|$)`),               // no trailing decimal
+	re(`\.[0-9]+0(e|$)`),        // no trailing zero in fraction
+	re(`^-?(0|[0-9]{2,})\..*e`), // exponential notation must have normalized mantissa
+	re(`e[0-9]`),                // positive exponent must be signed
+	re(`e[+-]0`),                // exponent must not have leading zeros
+	re(`e-[1-6]$`),              // not tiny enough for exponential notation
+	re(`e+(.|1.|20)$`),          // not big enough for exponential notation
+	re(`^-?0\.0000000`),         // too tiny, should use exponential notation
+	re(`^-?[0-9]{22}`),          // too big, should use exponential notation
+	re(`[1-9][0-9]{16}[1-9]`),   // too many significant digits in integer
+	re(`[1-9][0-9.]{17}[1-9]`),  // too many significant digits in decimal
+	// below here for float32 only
+	re(`[1-9][0-9]{8}[1-9]`),  // too many significant digits in integer
+	re(`[1-9][0-9.]{9}[1-9]`), // too many significant digits in decimal
+}
+
+func TestMarshalFloat(t *testing.T) {
+	t.Parallel()
+	nfail := 0
+	test := func(f float64, bits int) {
+		vf := interface{}(f)
+		if bits == 32 {
+			f = float64(float32(f)) // round
+			vf = float32(f)
+		}
+		bout, err := Marshal(vf)
+		if err != nil {
+			t.Errorf("Marshal(%T(%g)): %v", vf, vf, err)
+			nfail++
+			return
+		}
+		out := string(bout)
+
+		// result must convert back to the same float
+		g, err := strconv.ParseFloat(out, bits)
+		if err != nil {
+			t.Errorf("Marshal(%T(%g)) = %q, cannot parse back: %v", vf, vf, out, err)
+			nfail++
+			return
+		}
+		if f != g || fmt.Sprint(f) != fmt.Sprint(g) { // fmt.Sprint handles ±0
+			t.Errorf("Marshal(%T(%g)) = %q (is %g, not %g)", vf, vf, out, float32(g), vf)
+			nfail++
+			return
+		}
+
+		bad := badFloatREs
+		if bits == 64 {
+			bad = bad[:len(bad)-2]
+		}
+		for _, re := range bad {
+			if re.MatchString(out) {
+				t.Errorf("Marshal(%T(%g)) = %q, must not match /%s/", vf, vf, out, re)
+				nfail++
+				return
+			}
+		}
+	}
+
+	var (
+		bigger  = math.Inf(+1)
+		smaller = math.Inf(-1)
+	)
+
+	var digits = "1.2345678901234567890123"
+	for i := len(digits); i >= 2; i-- {
+		for exp := -30; exp <= 30; exp++ {
+			for _, sign := range "+-" {
+				for bits := 32; bits <= 64; bits += 32 {
+					s := fmt.Sprintf("%c%se%d", sign, digits[:i], exp)
+					f, err := strconv.ParseFloat(s, bits)
+					if err != nil {
+						log.Fatal(err)
+					}
+					next := math.Nextafter
+					if bits == 32 {
+						next = func(g, h float64) float64 {
+							return float64(math.Nextafter32(float32(g), float32(h)))
+						}
+					}
+					test(f, bits)
+					test(next(f, bigger), bits)
+					test(next(f, smaller), bits)
+					if nfail > 50 {
+						t.Fatalf("stopping test early")
+					}
+				}
+			}
+		}
+	}
+	test(0, 64)
+	test(math.Copysign(0, -1), 64)
+	test(0, 32)
+	test(math.Copysign(0, -1), 32)
+}
+
+func TestMarshalRawMessageValue(t *testing.T) {
+	type (
+		T1 struct {
+			M RawMessage `json:",omitempty"`
+		}
+		T2 struct {
+			M *RawMessage `json:",omitempty"`
+		}
+	)
+
+	var (
+		rawNil   = RawMessage(nil)
+		rawEmpty = RawMessage([]byte{})
+		rawText  = RawMessage([]byte(`"foo"`))
+	)
+
+	tests := []struct {
+		in   interface{}
+		want string
+		ok   bool
+	}{
+		// Test with nil RawMessage.
+		{rawNil, "null", true},
+		{&rawNil, "null", true},
+		{[]interface{}{rawNil}, "[null]", true},
+		{&[]interface{}{rawNil}, "[null]", true},
+		{[]interface{}{&rawNil}, "[null]", true},
+		{&[]interface{}{&rawNil}, "[null]", true},
+		{struct{ M RawMessage }{rawNil}, `{"M":null}`, true},
+		{&struct{ M RawMessage }{rawNil}, `{"M":null}`, true},
+		{struct{ M *RawMessage }{&rawNil}, `{"M":null}`, true},
+		{&struct{ M *RawMessage }{&rawNil}, `{"M":null}`, true},
+		{map[string]interface{}{"M": rawNil}, `{"M":null}`, true},
+		{&map[string]interface{}{"M": rawNil}, `{"M":null}`, true},
+		{map[string]interface{}{"M": &rawNil}, `{"M":null}`, true},
+		{&map[string]interface{}{"M": &rawNil}, `{"M":null}`, true},
+		{T1{rawNil}, "{}", true},
+		{T2{&rawNil}, `{"M":null}`, true},
+		{&T1{rawNil}, "{}", true},
+		{&T2{&rawNil}, `{"M":null}`, true},
+
+		// Test with empty, but non-nil, RawMessage.
+		{rawEmpty, "", false},
+		{&rawEmpty, "", false},
+		{[]interface{}{rawEmpty}, "", false},
+		{&[]interface{}{rawEmpty}, "", false},
+		{[]interface{}{&rawEmpty}, "", false},
+		{&[]interface{}{&rawEmpty}, "", false},
+		{struct{ X RawMessage }{rawEmpty}, "", false},
+		{&struct{ X RawMessage }{rawEmpty}, "", false},
+		{struct{ X *RawMessage }{&rawEmpty}, "", false},
+		{&struct{ X *RawMessage }{&rawEmpty}, "", false},
+		{map[string]interface{}{"nil": rawEmpty}, "", false},
+		{&map[string]interface{}{"nil": rawEmpty}, "", false},
+		{map[string]interface{}{"nil": &rawEmpty}, "", false},
+		{&map[string]interface{}{"nil": &rawEmpty}, "", false},
+		{T1{rawEmpty}, "{}", true},
+		{T2{&rawEmpty}, "", false},
+		{&T1{rawEmpty}, "{}", true},
+		{&T2{&rawEmpty}, "", false},
+
+		// Test with RawMessage with some text.
+		//
+		// The tests below marked with Issue6458 used to generate "ImZvbyI=" instead "foo".
+		// This behavior was intentionally changed in Go 1.8.
+		// See https://golang.org/issues/14493#issuecomment-255857318
+		{rawText, `"foo"`, true}, // Issue6458
+		{&rawText, `"foo"`, true},
+		{[]interface{}{rawText}, `["foo"]`, true},  // Issue6458
+		{&[]interface{}{rawText}, `["foo"]`, true}, // Issue6458
+		{[]interface{}{&rawText}, `["foo"]`, true},
+		{&[]interface{}{&rawText}, `["foo"]`, true},
+		{struct{ M RawMessage }{rawText}, `{"M":"foo"}`, true}, // Issue6458
+		{&struct{ M RawMessage }{rawText}, `{"M":"foo"}`, true},
+		{struct{ M *RawMessage }{&rawText}, `{"M":"foo"}`, true},
+		{&struct{ M *RawMessage }{&rawText}, `{"M":"foo"}`, true},
+		{map[string]interface{}{"M": rawText}, `{"M":"foo"}`, true},  // Issue6458
+		{&map[string]interface{}{"M": rawText}, `{"M":"foo"}`, true}, // Issue6458
+		{map[string]interface{}{"M": &rawText}, `{"M":"foo"}`, true},
+		{&map[string]interface{}{"M": &rawText}, `{"M":"foo"}`, true},
+		{T1{rawText}, `{"M":"foo"}`, true}, // Issue6458
+		{T2{&rawText}, `{"M":"foo"}`, true},
+		{&T1{rawText}, `{"M":"foo"}`, true},
+		{&T2{&rawText}, `{"M":"foo"}`, true},
+	}
+
+	for i, tt := range tests {
+		b, err := Marshal(tt.in)
+		if ok := (err == nil); ok != tt.ok {
+			if err != nil {
+				t.Errorf("test %d, unexpected failure: %v", i, err)
+			} else {
+				t.Errorf("test %d, unexpected success", i)
+			}
+		}
+		if got := string(b); got != tt.want {
+			t.Errorf("test %d, Marshal(%#v) = %q, want %q", i, tt.in, got, tt.want)
+		}
+	}
+}
+
+type marshalPanic struct{}
+
+func (marshalPanic) MarshalJSON() ([]byte, error) { panic(0xdead) }
+
+func TestMarshalPanic(t *testing.T) {
+	defer func() {
+		if got := recover(); !reflect.DeepEqual(got, 0xdead) {
+			t.Errorf("panic() = (%T)(%v), want 0xdead", got, got)
+		}
+	}()
+	Marshal(&marshalPanic{})
+	t.Error("Marshal should have panicked")
+}
+
+func TestMarshalUncommonFieldNames(t *testing.T) {
+	v := struct {
+		A0, À, Aβ int
+	}{}
+	b, err := Marshal(v)
+	if err != nil {
+		t.Fatal("Marshal:", err)
+	}
+	want := `{"A0":0,"À":0,"Aβ":0}`
+	got := string(b)
+	if got != want {
+		t.Fatalf("Marshal: got %s want %s", got, want)
+	}
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/example_marshaling_test.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/example_marshaling_test.go
new file mode 100644
index 0000000..7f15c74
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/example_marshaling_test.go
@@ -0,0 +1,73 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package json_test
+
+import (
+	"encoding/json"
+	"fmt"
+	"log"
+	"strings"
+)
+
+type Animal int
+
+const (
+	Unknown Animal = iota
+	Gopher
+	Zebra
+)
+
+func (a *Animal) UnmarshalJSON(b []byte) error {
+	var s string
+	if err := json.Unmarshal(b, &s); err != nil {
+		return err
+	}
+	switch strings.ToLower(s) {
+	default:
+		*a = Unknown
+	case "gopher":
+		*a = Gopher
+	case "zebra":
+		*a = Zebra
+	}
+
+	return nil
+}
+
+func (a Animal) MarshalJSON() ([]byte, error) {
+	var s string
+	switch a {
+	default:
+		s = "unknown"
+	case Gopher:
+		s = "gopher"
+	case Zebra:
+		s = "zebra"
+	}
+
+	return json.Marshal(s)
+}
+
+func Example_customMarshalJSON() {
+	blob := `["gopher","armadillo","zebra","unknown","gopher","bee","gopher","zebra"]`
+	var zoo []Animal
+	if err := json.Unmarshal([]byte(blob), &zoo); err != nil {
+		log.Fatal(err)
+	}
+
+	census := make(map[Animal]int)
+	for _, animal := range zoo {
+		census[animal] += 1
+	}
+
+	fmt.Printf("Zoo Census:\n* Gophers: %d\n* Zebras:  %d\n* Unknown: %d\n",
+		census[Gopher], census[Zebra], census[Unknown])
+
+	// Output:
+	// Zoo Census:
+	// * Gophers: 3
+	// * Zebras:  2
+	// * Unknown: 3
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/example_test.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/example_test.go
new file mode 100644
index 0000000..2031cba
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/example_test.go
@@ -0,0 +1,303 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package json_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"log"
+	"os"
+	"strings"
+)
+
+func ExampleMarshal() {
+	type ColorGroup struct {
+		ID     int
+		Name   string
+		Colors []string
+	}
+	group := ColorGroup{
+		ID:     1,
+		Name:   "Reds",
+		Colors: []string{"Crimson", "Red", "Ruby", "Maroon"},
+	}
+	b, err := json.Marshal(group)
+	if err != nil {
+		fmt.Println("error:", err)
+	}
+	os.Stdout.Write(b)
+	// Output:
+	// {"ID":1,"Name":"Reds","Colors":["Crimson","Red","Ruby","Maroon"]}
+}
+
+func ExampleUnmarshal() {
+	var jsonBlob = []byte(`[
+	{"Name": "Platypus", "Order": "Monotremata"},
+	{"Name": "Quoll",    "Order": "Dasyuromorphia"}
+]`)
+	type Animal struct {
+		Name  string
+		Order string
+	}
+	var animals []Animal
+	err := json.Unmarshal(jsonBlob, &animals)
+	if err != nil {
+		fmt.Println("error:", err)
+	}
+	fmt.Printf("%+v", animals)
+	// Output:
+	// [{Name:Platypus Order:Monotremata} {Name:Quoll Order:Dasyuromorphia}]
+}
+
+// This example uses a Decoder to decode a stream of distinct JSON values.
+func ExampleDecoder() {
+	const jsonStream = `
+	{"Name": "Ed", "Text": "Knock knock."}
+	{"Name": "Sam", "Text": "Who's there?"}
+	{"Name": "Ed", "Text": "Go fmt."}
+	{"Name": "Sam", "Text": "Go fmt who?"}
+	{"Name": "Ed", "Text": "Go fmt yourself!"}
+`
+	type Message struct {
+		Name, Text string
+	}
+	dec := json.NewDecoder(strings.NewReader(jsonStream))
+	for {
+		var m Message
+		if err := dec.Decode(&m); err == io.EOF {
+			break
+		} else if err != nil {
+			log.Fatal(err)
+		}
+		fmt.Printf("%s: %s\n", m.Name, m.Text)
+	}
+	// Output:
+	// Ed: Knock knock.
+	// Sam: Who's there?
+	// Ed: Go fmt.
+	// Sam: Go fmt who?
+	// Ed: Go fmt yourself!
+}
+
+// This example uses a Decoder to decode a stream of distinct JSON values.
+func ExampleDecoder_Token() {
+	const jsonStream = `
+	{"Message": "Hello", "Array": [1, 2, 3], "Null": null, "Number": 1.234}
+`
+	dec := json.NewDecoder(strings.NewReader(jsonStream))
+	for {
+		t, err := dec.Token()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			log.Fatal(err)
+		}
+		fmt.Printf("%T: %v", t, t)
+		if dec.More() {
+			fmt.Printf(" (more)")
+		}
+		fmt.Printf("\n")
+	}
+	// Output:
+	// json.Delim: { (more)
+	// string: Message (more)
+	// string: Hello (more)
+	// string: Array (more)
+	// json.Delim: [ (more)
+	// float64: 1 (more)
+	// float64: 2 (more)
+	// float64: 3
+	// json.Delim: ] (more)
+	// string: Null (more)
+	// <nil>: <nil> (more)
+	// string: Number (more)
+	// float64: 1.234
+	// json.Delim: }
+}
+
+// This example uses a Decoder to decode a streaming array of JSON objects.
+func ExampleDecoder_Decode_stream() {
+	const jsonStream = `
+	[
+		{"Name": "Ed", "Text": "Knock knock."},
+		{"Name": "Sam", "Text": "Who's there?"},
+		{"Name": "Ed", "Text": "Go fmt."},
+		{"Name": "Sam", "Text": "Go fmt who?"},
+		{"Name": "Ed", "Text": "Go fmt yourself!"}
+	]
+`
+	type Message struct {
+		Name, Text string
+	}
+	dec := json.NewDecoder(strings.NewReader(jsonStream))
+
+	// read open bracket
+	t, err := dec.Token()
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("%T: %v\n", t, t)
+
+	// while the array contains values
+	for dec.More() {
+		var m Message
+		// decode an array value (Message)
+		err := dec.Decode(&m)
+		if err != nil {
+			log.Fatal(err)
+		}
+
+		fmt.Printf("%v: %v\n", m.Name, m.Text)
+	}
+
+	// read closing bracket
+	t, err = dec.Token()
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("%T: %v\n", t, t)
+
+	// Output:
+	// json.Delim: [
+	// Ed: Knock knock.
+	// Sam: Who's there?
+	// Ed: Go fmt.
+	// Sam: Go fmt who?
+	// Ed: Go fmt yourself!
+	// json.Delim: ]
+
+}
+
+// This example uses RawMessage to delay parsing part of a JSON message.
+func ExampleRawMessage_unmarshal() {
+	type Color struct {
+		Space string
+		Point json.RawMessage // delay parsing until we know the color space
+	}
+	type RGB struct {
+		R uint8
+		G uint8
+		B uint8
+	}
+	type YCbCr struct {
+		Y  uint8
+		Cb int8
+		Cr int8
+	}
+
+	var j = []byte(`[
+	{"Space": "YCbCr", "Point": {"Y": 255, "Cb": 0, "Cr": -10}},
+	{"Space": "RGB",   "Point": {"R": 98, "G": 218, "B": 255}}
+]`)
+	var colors []Color
+	err := json.Unmarshal(j, &colors)
+	if err != nil {
+		log.Fatalln("error:", err)
+	}
+
+	for _, c := range colors {
+		var dst interface{}
+		switch c.Space {
+		case "RGB":
+			dst = new(RGB)
+		case "YCbCr":
+			dst = new(YCbCr)
+		}
+		err := json.Unmarshal(c.Point, dst)
+		if err != nil {
+			log.Fatalln("error:", err)
+		}
+		fmt.Println(c.Space, dst)
+	}
+	// Output:
+	// YCbCr &{255 0 -10}
+	// RGB &{98 218 255}
+}
+
+// This example uses RawMessage to use a precomputed JSON during marshal.
+func ExampleRawMessage_marshal() {
+	h := json.RawMessage(`{"precomputed": true}`)
+
+	c := struct {
+		Header *json.RawMessage `json:"header"`
+		Body   string           `json:"body"`
+	}{Header: &h, Body: "Hello Gophers!"}
+
+	b, err := json.MarshalIndent(&c, "", "\t")
+	if err != nil {
+		fmt.Println("error:", err)
+	}
+	os.Stdout.Write(b)
+
+	// Output:
+	// {
+	// 	"header": {
+	// 		"precomputed": true
+	// 	},
+	// 	"body": "Hello Gophers!"
+	// }
+}
+
+func ExampleIndent() {
+	type Road struct {
+		Name   string
+		Number int
+	}
+	roads := []Road{
+		{"Diamond Fork", 29},
+		{"Sheep Creek", 51},
+	}
+
+	b, err := json.Marshal(roads)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	var out bytes.Buffer
+	json.Indent(&out, b, "=", "\t")
+	out.WriteTo(os.Stdout)
+	// Output:
+	// [
+	// =	{
+	// =		"Name": "Diamond Fork",
+	// =		"Number": 29
+	// =	},
+	// =	{
+	// =		"Name": "Sheep Creek",
+	// =		"Number": 51
+	// =	}
+	// =]
+}
+
+func ExampleMarshalIndent() {
+	data := map[string]int{
+		"a": 1,
+		"b": 2,
+	}
+
+	json, err := json.MarshalIndent(data, "<prefix>", "<indent>")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Println(string(json))
+	// Output:
+	// {
+	// <prefix><indent>"a": 1,
+	// <prefix><indent>"b": 2
+	// <prefix>}
+}
+
+func ExampleValid() {
+	goodJSON := `{"example": 1}`
+	badJSON := `{"example":2:]}}`
+
+	fmt.Println(json.Valid([]byte(goodJSON)), json.Valid([]byte(badJSON)))
+	// Output:
+	// true false
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/example_text_marshaling_test.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/example_text_marshaling_test.go
new file mode 100644
index 0000000..04c7813
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/example_text_marshaling_test.go
@@ -0,0 +1,67 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package json_test
+
+import (
+	"encoding/json"
+	"fmt"
+	"log"
+	"strings"
+)
+
+type Size int
+
+const (
+	Unrecognized Size = iota
+	Small
+	Large
+)
+
+func (s *Size) UnmarshalText(text []byte) error {
+	switch strings.ToLower(string(text)) {
+	default:
+		*s = Unrecognized
+	case "small":
+		*s = Small
+	case "large":
+		*s = Large
+	}
+	return nil
+}
+
+func (s Size) MarshalText() ([]byte, error) {
+	var name string
+	switch s {
+	default:
+		name = "unrecognized"
+	case Small:
+		name = "small"
+	case Large:
+		name = "large"
+	}
+	return []byte(name), nil
+}
+
+func Example_textMarshalJSON() {
+	blob := `["small","regular","large","unrecognized","small","normal","small","large"]`
+	var inventory []Size
+	if err := json.Unmarshal([]byte(blob), &inventory); err != nil {
+		log.Fatal(err)
+	}
+
+	counts := make(map[Size]int)
+	for _, size := range inventory {
+		counts[size] += 1
+	}
+
+	fmt.Printf("Inventory Counts:\n* Small:        %d\n* Large:        %d\n* Unrecognized: %d\n",
+		counts[Small], counts[Large], counts[Unrecognized])
+
+	// Output:
+	// Inventory Counts:
+	// * Small:        3
+	// * Large:        2
+	// * Unrecognized: 3
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/fold.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/fold.go
new file mode 100644
index 0000000..9e17012
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/fold.go
@@ -0,0 +1,143 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package json
+
+import (
+	"bytes"
+	"unicode/utf8"
+)
+
+const (
+	caseMask     = ^byte(0x20) // Mask to ignore case in ASCII.
+	kelvin       = '\u212a'
+	smallLongEss = '\u017f'
+)
+
+// foldFunc returns one of four different case folding equivalence
+// functions, from most general (and slow) to fastest:
+//
+// 1) bytes.EqualFold, if the key s contains any non-ASCII UTF-8
+// 2) equalFoldRight, if s contains special folding ASCII ('k', 'K', 's', 'S')
+// 3) asciiEqualFold, no special, but includes non-letters (including _)
+// 4) simpleLetterEqualFold, no specials, no non-letters.
+//
+// The letters S and K are special because they map to 3 runes, not just 2:
+//  * S maps to s and to U+017F 'ſ' Latin small letter long s
+//  * k maps to K and to U+212A 'K' Kelvin sign
+// See https://play.golang.org/p/tTxjOc0OGo
+//
+// The returned function is specialized for matching against s and
+// should only be given s. It's not curried for performance reasons.
+func foldFunc(s []byte) func(s, t []byte) bool {
+	nonLetter := false
+	special := false // special letter
+	for _, b := range s {
+		if b >= utf8.RuneSelf {
+			return bytes.EqualFold
+		}
+		upper := b & caseMask
+		if upper < 'A' || upper > 'Z' {
+			nonLetter = true
+		} else if upper == 'K' || upper == 'S' {
+			// See above for why these letters are special.
+			special = true
+		}
+	}
+	if special {
+		return equalFoldRight
+	}
+	if nonLetter {
+		return asciiEqualFold
+	}
+	return simpleLetterEqualFold
+}
+
+// equalFoldRight is a specialization of bytes.EqualFold when s is
+// known to be all ASCII (including punctuation), but contains an 's',
+// 'S', 'k', or 'K', requiring a Unicode fold on the bytes in t.
+// See comments on foldFunc.
+func equalFoldRight(s, t []byte) bool {
+	for _, sb := range s {
+		if len(t) == 0 {
+			return false
+		}
+		tb := t[0]
+		if tb < utf8.RuneSelf {
+			if sb != tb {
+				sbUpper := sb & caseMask
+				if 'A' <= sbUpper && sbUpper <= 'Z' {
+					if sbUpper != tb&caseMask {
+						return false
+					}
+				} else {
+					return false
+				}
+			}
+			t = t[1:]
+			continue
+		}
+		// sb is ASCII and t is not. t must be either kelvin
+		// sign or long s; sb must be s, S, k, or K.
+		tr, size := utf8.DecodeRune(t)
+		switch sb {
+		case 's', 'S':
+			if tr != smallLongEss {
+				return false
+			}
+		case 'k', 'K':
+			if tr != kelvin {
+				return false
+			}
+		default:
+			return false
+		}
+		t = t[size:]
+
+	}
+	if len(t) > 0 {
+		return false
+	}
+	return true
+}
+
+// asciiEqualFold is a specialization of bytes.EqualFold for use when
+// s is all ASCII (but may contain non-letters) and contains no
+// special-folding letters.
+// See comments on foldFunc.
+func asciiEqualFold(s, t []byte) bool {
+	if len(s) != len(t) {
+		return false
+	}
+	for i, sb := range s {
+		tb := t[i]
+		if sb == tb {
+			continue
+		}
+		if ('a' <= sb && sb <= 'z') || ('A' <= sb && sb <= 'Z') {
+			if sb&caseMask != tb&caseMask {
+				return false
+			}
+		} else {
+			return false
+		}
+	}
+	return true
+}
+
+// simpleLetterEqualFold is a specialization of bytes.EqualFold for
+// use when s is all ASCII letters (no underscores, etc) and also
+// doesn't contain 'k', 'K', 's', or 'S'.
+// See comments on foldFunc.
+func simpleLetterEqualFold(s, t []byte) bool {
+	if len(s) != len(t) {
+		return false
+	}
+	for i, b := range s {
+		if b&caseMask != t[i]&caseMask {
+			return false
+		}
+	}
+	return true
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/fold_test.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/fold_test.go
new file mode 100644
index 0000000..9fb9464
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/fold_test.go
@@ -0,0 +1,116 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package json
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+	"unicode/utf8"
+)
+
+var foldTests = []struct {
+	fn   func(s, t []byte) bool
+	s, t string
+	want bool
+}{
+	{equalFoldRight, "", "", true},
+	{equalFoldRight, "a", "a", true},
+	{equalFoldRight, "", "a", false},
+	{equalFoldRight, "a", "", false},
+	{equalFoldRight, "a", "A", true},
+	{equalFoldRight, "AB", "ab", true},
+	{equalFoldRight, "AB", "ac", false},
+	{equalFoldRight, "sbkKc", "ſbKKc", true},
+	{equalFoldRight, "SbKkc", "ſbKKc", true},
+	{equalFoldRight, "SbKkc", "ſbKK", false},
+	{equalFoldRight, "e", "é", false},
+	{equalFoldRight, "s", "S", true},
+
+	{simpleLetterEqualFold, "", "", true},
+	{simpleLetterEqualFold, "abc", "abc", true},
+	{simpleLetterEqualFold, "abc", "ABC", true},
+	{simpleLetterEqualFold, "abc", "ABCD", false},
+	{simpleLetterEqualFold, "abc", "xxx", false},
+
+	{asciiEqualFold, "a_B", "A_b", true},
+	{asciiEqualFold, "aa@", "aa`", false}, // verify 0x40 and 0x60 aren't case-equivalent
+}
+
+func TestFold(t *testing.T) {
+	for i, tt := range foldTests {
+		if got := tt.fn([]byte(tt.s), []byte(tt.t)); got != tt.want {
+			t.Errorf("%d. %q, %q = %v; want %v", i, tt.s, tt.t, got, tt.want)
+		}
+		truth := strings.EqualFold(tt.s, tt.t)
+		if truth != tt.want {
+			t.Errorf("strings.EqualFold doesn't agree with case %d", i)
+		}
+	}
+}
+
+func TestFoldAgainstUnicode(t *testing.T) {
+	const bufSize = 5
+	buf1 := make([]byte, 0, bufSize)
+	buf2 := make([]byte, 0, bufSize)
+	var runes []rune
+	for i := 0x20; i <= 0x7f; i++ {
+		runes = append(runes, rune(i))
+	}
+	runes = append(runes, kelvin, smallLongEss)
+
+	funcs := []struct {
+		name   string
+		fold   func(s, t []byte) bool
+		letter bool // must be ASCII letter
+		simple bool // must be simple ASCII letter (not 'S' or 'K')
+	}{
+		{
+			name: "equalFoldRight",
+			fold: equalFoldRight,
+		},
+		{
+			name:   "asciiEqualFold",
+			fold:   asciiEqualFold,
+			simple: true,
+		},
+		{
+			name:   "simpleLetterEqualFold",
+			fold:   simpleLetterEqualFold,
+			simple: true,
+			letter: true,
+		},
+	}
+
+	for _, ff := range funcs {
+		for _, r := range runes {
+			if r >= utf8.RuneSelf {
+				continue
+			}
+			if ff.letter && !isASCIILetter(byte(r)) {
+				continue
+			}
+			if ff.simple && (r == 's' || r == 'S' || r == 'k' || r == 'K') {
+				continue
+			}
+			for _, r2 := range runes {
+				buf1 := append(buf1[:0], 'x')
+				buf2 := append(buf2[:0], 'x')
+				buf1 = buf1[:1+utf8.EncodeRune(buf1[1:bufSize], r)]
+				buf2 = buf2[:1+utf8.EncodeRune(buf2[1:bufSize], r2)]
+				buf1 = append(buf1, 'x')
+				buf2 = append(buf2, 'x')
+				want := bytes.EqualFold(buf1, buf2)
+				if got := ff.fold(buf1, buf2); got != want {
+					t.Errorf("%s(%q, %q) = %v; want %v", ff.name, buf1, buf2, got, want)
+				}
+			}
+		}
+	}
+}
+
+func isASCIILetter(b byte) bool {
+	return ('A' <= b && b <= 'Z') || ('a' <= b && b <= 'z')
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/indent.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/indent.go
new file mode 100644
index 0000000..fba1954
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/indent.go
@@ -0,0 +1,141 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package json
+
+import "bytes"
+
+// Compact appends to dst the JSON-encoded src with
+// insignificant space characters elided.
+func Compact(dst *bytes.Buffer, src []byte) error {
+	return compact(dst, src, false)
+}
+
+func compact(dst *bytes.Buffer, src []byte, escape bool) error {
+	origLen := dst.Len()
+	var scan scanner
+	scan.reset()
+	start := 0
+	for i, c := range src {
+		if escape && (c == '<' || c == '>' || c == '&') {
+			if start < i {
+				dst.Write(src[start:i])
+			}
+			dst.WriteString(`\u00`)
+			dst.WriteByte(hex[c>>4])
+			dst.WriteByte(hex[c&0xF])
+			start = i + 1
+		}
+		// Convert U+2028 and U+2029 (E2 80 A8 and E2 80 A9).
+		if c == 0xE2 && i+2 < len(src) && src[i+1] == 0x80 && src[i+2]&^1 == 0xA8 {
+			if start < i {
+				dst.Write(src[start:i])
+			}
+			dst.WriteString(`\u202`)
+			dst.WriteByte(hex[src[i+2]&0xF])
+			start = i + 3
+		}
+		v := scan.step(&scan, c)
+		if v >= scanSkipSpace {
+			if v == scanError {
+				break
+			}
+			if start < i {
+				dst.Write(src[start:i])
+			}
+			start = i + 1
+		}
+	}
+	if scan.eof() == scanError {
+		dst.Truncate(origLen)
+		return scan.err
+	}
+	if start < len(src) {
+		dst.Write(src[start:])
+	}
+	return nil
+}
+
+func newline(dst *bytes.Buffer, prefix, indent string, depth int) {
+	dst.WriteByte('\n')
+	dst.WriteString(prefix)
+	for i := 0; i < depth; i++ {
+		dst.WriteString(indent)
+	}
+}
+
+// Indent appends to dst an indented form of the JSON-encoded src.
+// Each element in a JSON object or array begins on a new,
+// indented line beginning with prefix followed by one or more
+// copies of indent according to the indentation nesting.
+// The data appended to dst does not begin with the prefix nor
+// any indentation, to make it easier to embed inside other formatted JSON data.
+// Although leading space characters (space, tab, carriage return, newline)
+// at the beginning of src are dropped, trailing space characters
+// at the end of src are preserved and copied to dst.
+// For example, if src has no trailing spaces, neither will dst;
+// if src ends in a trailing newline, so will dst.
+func Indent(dst *bytes.Buffer, src []byte, prefix, indent string) error {
+	origLen := dst.Len()
+	var scan scanner
+	scan.reset()
+	needIndent := false
+	depth := 0
+	for _, c := range src {
+		scan.bytes++
+		v := scan.step(&scan, c)
+		if v == scanSkipSpace {
+			continue
+		}
+		if v == scanError {
+			break
+		}
+		if needIndent && v != scanEndObject && v != scanEndArray {
+			needIndent = false
+			depth++
+			newline(dst, prefix, indent, depth)
+		}
+
+		// Emit semantically uninteresting bytes
+		// (in particular, punctuation in strings) unmodified.
+		if v == scanContinue {
+			dst.WriteByte(c)
+			continue
+		}
+
+		// Add spacing around real punctuation.
+		switch c {
+		case '{', '[':
+			// delay indent so that empty object and array are formatted as {} and [].
+			needIndent = true
+			dst.WriteByte(c)
+
+		case ',':
+			dst.WriteByte(c)
+			newline(dst, prefix, indent, depth)
+
+		case ':':
+			dst.WriteByte(c)
+			dst.WriteByte(' ')
+
+		case '}', ']':
+			if needIndent {
+				// suppress indent in empty object/array
+				needIndent = false
+			} else {
+				depth--
+				newline(dst, prefix, indent, depth)
+			}
+			dst.WriteByte(c)
+
+		default:
+			dst.WriteByte(c)
+		}
+	}
+	if scan.eof() == scanError {
+		dst.Truncate(origLen)
+		return scan.err
+	}
+	return nil
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/number_test.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/number_test.go
new file mode 100644
index 0000000..cc67018
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/number_test.go
@@ -0,0 +1,133 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package json
+
+import (
+	"regexp"
+	"testing"
+)
+
+func TestNumberIsValid(t *testing.T) {
+	// From: https://stackoverflow.com/a/13340826
+	var jsonNumberRegexp = regexp.MustCompile(`^-?(?:0|[1-9]\d*)(?:\.\d+)?(?:[eE][+-]?\d+)?$`)
+
+	validTests := []string{
+		"0",
+		"-0",
+		"1",
+		"-1",
+		"0.1",
+		"-0.1",
+		"1234",
+		"-1234",
+		"12.34",
+		"-12.34",
+		"12E0",
+		"12E1",
+		"12e34",
+		"12E-0",
+		"12e+1",
+		"12e-34",
+		"-12E0",
+		"-12E1",
+		"-12e34",
+		"-12E-0",
+		"-12e+1",
+		"-12e-34",
+		"1.2E0",
+		"1.2E1",
+		"1.2e34",
+		"1.2E-0",
+		"1.2e+1",
+		"1.2e-34",
+		"-1.2E0",
+		"-1.2E1",
+		"-1.2e34",
+		"-1.2E-0",
+		"-1.2e+1",
+		"-1.2e-34",
+		"0E0",
+		"0E1",
+		"0e34",
+		"0E-0",
+		"0e+1",
+		"0e-34",
+		"-0E0",
+		"-0E1",
+		"-0e34",
+		"-0E-0",
+		"-0e+1",
+		"-0e-34",
+	}
+
+	for _, test := range validTests {
+		if !isValidNumber(test) {
+			t.Errorf("%s should be valid", test)
+		}
+
+		var f float64
+		if err := Unmarshal([]byte(test), &f); err != nil {
+			t.Errorf("%s should be valid but Unmarshal failed: %v", test, err)
+		}
+
+		if !jsonNumberRegexp.MatchString(test) {
+			t.Errorf("%s should be valid but regexp does not match", test)
+		}
+	}
+
+	invalidTests := []string{
+		"",
+		"invalid",
+		"1.0.1",
+		"1..1",
+		"-1-2",
+		"012a42",
+		"01.2",
+		"012",
+		"12E12.12",
+		"1e2e3",
+		"1e+-2",
+		"1e--23",
+		"1e",
+		"e1",
+		"1e+",
+		"1ea",
+		"1a",
+		"1.a",
+		"1.",
+		"01",
+		"1.e1",
+	}
+
+	for _, test := range invalidTests {
+		if isValidNumber(test) {
+			t.Errorf("%s should be invalid", test)
+		}
+
+		var f float64
+		if err := Unmarshal([]byte(test), &f); err == nil {
+			t.Errorf("%s should be invalid but unmarshal wrote %v", test, f)
+		}
+
+		if jsonNumberRegexp.MatchString(test) {
+			t.Errorf("%s should be invalid but matches regexp", test)
+		}
+	}
+}
+
+func BenchmarkNumberIsValid(b *testing.B) {
+	s := "-61657.61667E+61673"
+	for i := 0; i < b.N; i++ {
+		isValidNumber(s)
+	}
+}
+
+func BenchmarkNumberIsValidRegexp(b *testing.B) {
+	var jsonNumberRegexp = regexp.MustCompile(`^-?(?:0|[1-9]\d*)(?:\.\d+)?(?:[eE][+-]?\d+)?$`)
+	s := "-61657.61667E+61673"
+	for i := 0; i < b.N; i++ {
+		jsonNumberRegexp.MatchString(s)
+	}
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/scanner.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/scanner.go
new file mode 100644
index 0000000..8857224
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/scanner.go
@@ -0,0 +1,573 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package json
+
+// JSON value parser state machine.
+// Just about at the limit of what is reasonable to write by hand.
+// Some parts are a bit tedious, but overall it nicely factors out the
+// otherwise common code from the multiple scanning functions
+// in this package (Compact, Indent, checkValid, etc).
+//
+// This file starts with two simple examples using the scanner
+// before diving into the scanner itself.
+
+import "strconv"
+
+// Valid reports whether data is a valid JSON encoding.
+func Valid(data []byte) bool {
+	return checkValid(data, &scanner{}) == nil
+}
+
+// checkValid verifies that data is valid JSON-encoded data.
+// scan is passed in for use by checkValid to avoid an allocation.
+func checkValid(data []byte, scan *scanner) error {
+	scan.reset()
+	for _, c := range data {
+		scan.bytes++
+		if scan.step(scan, c) == scanError {
+			return scan.err
+		}
+	}
+	if scan.eof() == scanError {
+		return scan.err
+	}
+	return nil
+}
+
+// A SyntaxError is a description of a JSON syntax error.
+type SyntaxError struct {
+	msg    string // description of error
+	Offset int64  // error occurred after reading Offset bytes
+}
+
+func (e *SyntaxError) Error() string { return e.msg }
+
+// A scanner is a JSON scanning state machine.
+// Callers call scan.reset() and then pass bytes in one at a time
+// by calling scan.step(&scan, c) for each byte.
+// The return value, referred to as an opcode, tells the
+// caller about significant parsing events like beginning
+// and ending literals, objects, and arrays, so that the
+// caller can follow along if it wishes.
+// The return value scanEnd indicates that a single top-level
+// JSON value has been completed, *before* the byte that
+// just got passed in.  (The indication must be delayed in order
+// to recognize the end of numbers: is 123 a whole value or
+// the beginning of 12345e+6?).
+type scanner struct {
+	// The step is a func to be called to execute the next transition.
+	// Also tried using an integer constant and a single func
+	// with a switch, but using the func directly was 10% faster
+	// on a 64-bit Mac Mini, and it's nicer to read.
+	step func(*scanner, byte) int
+
+	// Reached end of top-level value.
+	endTop bool
+
+	// Stack of what we're in the middle of - array values, object keys, object values.
+	parseState []int
+
+	// Error that happened, if any.
+	err error
+
+	// total bytes consumed, updated by decoder.Decode
+	bytes int64
+}
+
+// These values are returned by the state transition functions
+// assigned to scanner.state and the method scanner.eof.
+// They give details about the current state of the scan that
+// callers might be interested to know about.
+// It is okay to ignore the return value of any particular
+// call to scanner.state: if one call returns scanError,
+// every subsequent call will return scanError too.
+const (
+	// Continue.
+	scanContinue     = iota // uninteresting byte
+	scanBeginLiteral        // end implied by next result != scanContinue
+	scanBeginObject         // begin object
+	scanObjectKey           // just finished object key (string)
+	scanObjectValue         // just finished non-last object value
+	scanEndObject           // end object (implies scanObjectValue if possible)
+	scanBeginArray          // begin array
+	scanArrayValue          // just finished array value
+	scanEndArray            // end array (implies scanArrayValue if possible)
+	scanSkipSpace           // space byte; can skip; known to be last "continue" result
+
+	// Stop.
+	scanEnd   // top-level value ended *before* this byte; known to be first "stop" result
+	scanError // hit an error, scanner.err.
+)
+
+// These values are stored in the parseState stack.
+// They give the current state of a composite value
+// being scanned. If the parser is inside a nested value
+// the parseState describes the nested state, outermost at entry 0.
+const (
+	parseObjectKey   = iota // parsing object key (before colon)
+	parseObjectValue        // parsing object value (after colon)
+	parseArrayValue         // parsing array value
+)
+
+// reset prepares the scanner for use.
+// It must be called before calling s.step.
+func (s *scanner) reset() {
+	s.step = stateBeginValue
+	s.parseState = s.parseState[0:0]
+	s.err = nil
+	s.endTop = false
+}
+
+// eof tells the scanner that the end of input has been reached.
+// It returns a scan status just as s.step does.
+func (s *scanner) eof() int {
+	if s.err != nil {
+		return scanError
+	}
+	if s.endTop {
+		return scanEnd
+	}
+	s.step(s, ' ')
+	if s.endTop {
+		return scanEnd
+	}
+	if s.err == nil {
+		s.err = &SyntaxError{"unexpected end of JSON input", s.bytes}
+	}
+	return scanError
+}
+
+// pushParseState pushes a new parse state p onto the parse stack.
+func (s *scanner) pushParseState(p int) {
+	s.parseState = append(s.parseState, p)
+}
+
+// popParseState pops a parse state (already obtained) off the stack
+// and updates s.step accordingly.
+func (s *scanner) popParseState() {
+	n := len(s.parseState) - 1
+	s.parseState = s.parseState[0:n]
+	if n == 0 {
+		s.step = stateEndTop
+		s.endTop = true
+	} else {
+		s.step = stateEndValue
+	}
+}
+
+func isSpace(c byte) bool {
+	return c == ' ' || c == '\t' || c == '\r' || c == '\n'
+}
+
+// stateBeginValueOrEmpty is the state after reading `[`.
+func stateBeginValueOrEmpty(s *scanner, c byte) int {
+	if c <= ' ' && isSpace(c) {
+		return scanSkipSpace
+	}
+	if c == ']' {
+		return stateEndValue(s, c)
+	}
+	return stateBeginValue(s, c)
+}
+
+// stateBeginValue is the state at the beginning of the input.
+func stateBeginValue(s *scanner, c byte) int {
+	if c <= ' ' && isSpace(c) {
+		return scanSkipSpace
+	}
+	switch c {
+	case '{':
+		s.step = stateBeginStringOrEmpty
+		s.pushParseState(parseObjectKey)
+		return scanBeginObject
+	case '[':
+		s.step = stateBeginValueOrEmpty
+		s.pushParseState(parseArrayValue)
+		return scanBeginArray
+	case '"':
+		s.step = stateInString
+		return scanBeginLiteral
+	case '-':
+		s.step = stateNeg
+		return scanBeginLiteral
+	case '0': // beginning of 0.123
+		s.step = state0
+		return scanBeginLiteral
+	case 't': // beginning of true
+		s.step = stateT
+		return scanBeginLiteral
+	case 'f': // beginning of false
+		s.step = stateF
+		return scanBeginLiteral
+	case 'n': // beginning of null
+		s.step = stateN
+		return scanBeginLiteral
+	}
+	if '1' <= c && c <= '9' { // beginning of 1234.5
+		s.step = state1
+		return scanBeginLiteral
+	}
+	return s.error(c, "looking for beginning of value")
+}
+
+// stateBeginStringOrEmpty is the state after reading `{`.
+func stateBeginStringOrEmpty(s *scanner, c byte) int {
+	if c <= ' ' && isSpace(c) {
+		return scanSkipSpace
+	}
+	if c == '}' {
+		n := len(s.parseState)
+		s.parseState[n-1] = parseObjectValue
+		return stateEndValue(s, c)
+	}
+	return stateBeginString(s, c)
+}
+
+// stateBeginString is the state after reading `{"key": value,`.
+func stateBeginString(s *scanner, c byte) int {
+	if c <= ' ' && isSpace(c) {
+		return scanSkipSpace
+	}
+	if c == '"' {
+		s.step = stateInString
+		return scanBeginLiteral
+	}
+	return s.error(c, "looking for beginning of object key string")
+}
+
+// stateEndValue is the state after completing a value,
+// such as after reading `{}` or `true` or `["x"`.
+func stateEndValue(s *scanner, c byte) int {
+	n := len(s.parseState)
+	if n == 0 {
+		// Completed top-level before the current byte.
+		s.step = stateEndTop
+		s.endTop = true
+		return stateEndTop(s, c)
+	}
+	if c <= ' ' && isSpace(c) {
+		s.step = stateEndValue
+		return scanSkipSpace
+	}
+	ps := s.parseState[n-1]
+	switch ps {
+	case parseObjectKey:
+		if c == ':' {
+			s.parseState[n-1] = parseObjectValue
+			s.step = stateBeginValue
+			return scanObjectKey
+		}
+		return s.error(c, "after object key")
+	case parseObjectValue:
+		if c == ',' {
+			s.parseState[n-1] = parseObjectKey
+			s.step = stateBeginString
+			return scanObjectValue
+		}
+		if c == '}' {
+			s.popParseState()
+			return scanEndObject
+		}
+		return s.error(c, "after object key:value pair")
+	case parseArrayValue:
+		if c == ',' {
+			s.step = stateBeginValue
+			return scanArrayValue
+		}
+		if c == ']' {
+			s.popParseState()
+			return scanEndArray
+		}
+		return s.error(c, "after array element")
+	}
+	return s.error(c, "")
+}
+
+// stateEndTop is the state after finishing the top-level value,
+// such as after reading `{}` or `[1,2,3]`.
+// Only space characters should be seen now.
+func stateEndTop(s *scanner, c byte) int {
+	if !isSpace(c) {
+		// Complain about non-space byte on next call.
+		s.error(c, "after top-level value")
+	}
+	return scanEnd
+}
+
+// stateInString is the state after reading `"`.
+func stateInString(s *scanner, c byte) int {
+	if c == '"' {
+		s.step = stateEndValue
+		return scanContinue
+	}
+	if c == '\\' {
+		s.step = stateInStringEsc
+		return scanContinue
+	}
+	if c < 0x20 {
+		return s.error(c, "in string literal")
+	}
+	return scanContinue
+}
+
+// stateInStringEsc is the state after reading `"\` during a quoted string.
+func stateInStringEsc(s *scanner, c byte) int {
+	switch c {
+	case 'b', 'f', 'n', 'r', 't', '\\', '/', '"':
+		s.step = stateInString
+		return scanContinue
+	case 'u':
+		s.step = stateInStringEscU
+		return scanContinue
+	}
+	return s.error(c, "in string escape code")
+}
+
+// stateInStringEscU is the state after reading `"\u` during a quoted string.
+func stateInStringEscU(s *scanner, c byte) int {
+	if '0' <= c && c <= '9' || 'a' <= c && c <= 'f' || 'A' <= c && c <= 'F' {
+		s.step = stateInStringEscU1
+		return scanContinue
+	}
+	// numbers
+	return s.error(c, "in \\u hexadecimal character escape")
+}
+
+// stateInStringEscU1 is the state after reading `"\u1` during a quoted string.
+func stateInStringEscU1(s *scanner, c byte) int {
+	if '0' <= c && c <= '9' || 'a' <= c && c <= 'f' || 'A' <= c && c <= 'F' {
+		s.step = stateInStringEscU12
+		return scanContinue
+	}
+	// numbers
+	return s.error(c, "in \\u hexadecimal character escape")
+}
+
+// stateInStringEscU12 is the state after reading `"\u12` during a quoted string.
+func stateInStringEscU12(s *scanner, c byte) int {
+	if '0' <= c && c <= '9' || 'a' <= c && c <= 'f' || 'A' <= c && c <= 'F' {
+		s.step = stateInStringEscU123
+		return scanContinue
+	}
+	// numbers
+	return s.error(c, "in \\u hexadecimal character escape")
+}
+
+// stateInStringEscU123 is the state after reading `"\u123` during a quoted string.
+func stateInStringEscU123(s *scanner, c byte) int {
+	if '0' <= c && c <= '9' || 'a' <= c && c <= 'f' || 'A' <= c && c <= 'F' {
+		s.step = stateInString
+		return scanContinue
+	}
+	// numbers
+	return s.error(c, "in \\u hexadecimal character escape")
+}
+
+// stateNeg is the state after reading `-` during a number.
+func stateNeg(s *scanner, c byte) int {
+	if c == '0' {
+		s.step = state0
+		return scanContinue
+	}
+	if '1' <= c && c <= '9' {
+		s.step = state1
+		return scanContinue
+	}
+	return s.error(c, "in numeric literal")
+}
+
+// state1 is the state after reading a non-zero integer during a number,
+// such as after reading `1` or `100` but not `0`.
+func state1(s *scanner, c byte) int {
+	if '0' <= c && c <= '9' {
+		s.step = state1
+		return scanContinue
+	}
+	return state0(s, c)
+}
+
+// state0 is the state after reading `0` during a number.
+func state0(s *scanner, c byte) int {
+	if c == '.' {
+		s.step = stateDot
+		return scanContinue
+	}
+	if c == 'e' || c == 'E' {
+		s.step = stateE
+		return scanContinue
+	}
+	return stateEndValue(s, c)
+}
+
+// stateDot is the state after reading the integer and decimal point in a number,
+// such as after reading `1.`.
+func stateDot(s *scanner, c byte) int {
+	if '0' <= c && c <= '9' {
+		s.step = stateDot0
+		return scanContinue
+	}
+	return s.error(c, "after decimal point in numeric literal")
+}
+
+// stateDot0 is the state after reading the integer, decimal point, and subsequent
+// digits of a number, such as after reading `3.14`.
+func stateDot0(s *scanner, c byte) int {
+	if '0' <= c && c <= '9' {
+		return scanContinue
+	}
+	if c == 'e' || c == 'E' {
+		s.step = stateE
+		return scanContinue
+	}
+	return stateEndValue(s, c)
+}
+
+// stateE is the state after reading the mantissa and e in a number,
+// such as after reading `314e` or `0.314e`.
+func stateE(s *scanner, c byte) int {
+	if c == '+' || c == '-' {
+		s.step = stateESign
+		return scanContinue
+	}
+	return stateESign(s, c)
+}
+
+// stateESign is the state after reading the mantissa, e, and sign in a number,
+// such as after reading `314e-` or `0.314e+`.
+func stateESign(s *scanner, c byte) int {
+	if '0' <= c && c <= '9' {
+		s.step = stateE0
+		return scanContinue
+	}
+	return s.error(c, "in exponent of numeric literal")
+}
+
+// stateE0 is the state after reading the mantissa, e, optional sign,
+// and at least one digit of the exponent in a number,
+// such as after reading `314e-2` or `0.314e+1` or `3.14e0`.
+func stateE0(s *scanner, c byte) int {
+	if '0' <= c && c <= '9' {
+		return scanContinue
+	}
+	return stateEndValue(s, c)
+}
+
+// stateT is the state after reading `t`.
+func stateT(s *scanner, c byte) int {
+	if c == 'r' {
+		s.step = stateTr
+		return scanContinue
+	}
+	return s.error(c, "in literal true (expecting 'r')")
+}
+
+// stateTr is the state after reading `tr`.
+func stateTr(s *scanner, c byte) int {
+	if c == 'u' {
+		s.step = stateTru
+		return scanContinue
+	}
+	return s.error(c, "in literal true (expecting 'u')")
+}
+
+// stateTru is the state after reading `tru`.
+func stateTru(s *scanner, c byte) int {
+	if c == 'e' {
+		s.step = stateEndValue
+		return scanContinue
+	}
+	return s.error(c, "in literal true (expecting 'e')")
+}
+
+// stateF is the state after reading `f`.
+func stateF(s *scanner, c byte) int {
+	if c == 'a' {
+		s.step = stateFa
+		return scanContinue
+	}
+	return s.error(c, "in literal false (expecting 'a')")
+}
+
+// stateFa is the state after reading `fa`.
+func stateFa(s *scanner, c byte) int {
+	if c == 'l' {
+		s.step = stateFal
+		return scanContinue
+	}
+	return s.error(c, "in literal false (expecting 'l')")
+}
+
+// stateFal is the state after reading `fal`.
+func stateFal(s *scanner, c byte) int {
+	if c == 's' {
+		s.step = stateFals
+		return scanContinue
+	}
+	return s.error(c, "in literal false (expecting 's')")
+}
+
+// stateFals is the state after reading `fals`.
+func stateFals(s *scanner, c byte) int {
+	if c == 'e' {
+		s.step = stateEndValue
+		return scanContinue
+	}
+	return s.error(c, "in literal false (expecting 'e')")
+}
+
+// stateN is the state after reading `n`.
+func stateN(s *scanner, c byte) int {
+	if c == 'u' {
+		s.step = stateNu
+		return scanContinue
+	}
+	return s.error(c, "in literal null (expecting 'u')")
+}
+
+// stateNu is the state after reading `nu`.
+func stateNu(s *scanner, c byte) int {
+	if c == 'l' {
+		s.step = stateNul
+		return scanContinue
+	}
+	return s.error(c, "in literal null (expecting 'l')")
+}
+
+// stateNul is the state after reading `nul`.
+func stateNul(s *scanner, c byte) int {
+	if c == 'l' {
+		s.step = stateEndValue
+		return scanContinue
+	}
+	return s.error(c, "in literal null (expecting 'l')")
+}
+
+// stateError is the state after reaching a syntax error,
+// such as after reading `[1}` or `5.1.2`.
+func stateError(s *scanner, c byte) int {
+	return scanError
+}
+
+// error records an error and switches to the error state.
+func (s *scanner) error(c byte, context string) int {
+	s.step = stateError
+	s.err = &SyntaxError{"invalid character " + quoteChar(c) + " " + context, s.bytes}
+	return scanError
+}
+
+// quoteChar formats c as a quoted character literal
+func quoteChar(c byte) string {
+	// special cases - different from quoted strings
+	if c == '\'' {
+		return `'\''`
+	}
+	if c == '"' {
+		return `'"'`
+	}
+
+	// use quoted string with different quotation marks
+	s := strconv.Quote(string(c))
+	return "'" + s[1:len(s)-1] + "'"
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/scanner_test.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/scanner_test.go
new file mode 100644
index 0000000..6cdbe7d
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/scanner_test.go
@@ -0,0 +1,300 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package json
+
+import (
+	"bytes"
+	"math"
+	"math/rand"
+	"reflect"
+	"testing"
+)
+
+var validTests = []struct {
+	data string
+	ok   bool
+}{
+	{`foo`, false},
+	{`}{`, false},
+	{`{]`, false},
+	{`{}`, true},
+	{`{"foo":"bar"}`, true},
+	{`{"foo":"bar","bar":{"baz":["qux"]}}`, true},
+}
+
+func TestValid(t *testing.T) {
+	for _, tt := range validTests {
+		if ok := Valid([]byte(tt.data)); ok != tt.ok {
+			t.Errorf("Valid(%#q) = %v, want %v", tt.data, ok, tt.ok)
+		}
+	}
+}
+
+// Tests of simple examples.
+
+type example struct {
+	compact string
+	indent  string
+}
+
+var examples = []example{
+	{`1`, `1`},
+	{`{}`, `{}`},
+	{`[]`, `[]`},
+	{`{"":2}`, "{\n\t\"\": 2\n}"},
+	{`[3]`, "[\n\t3\n]"},
+	{`[1,2,3]`, "[\n\t1,\n\t2,\n\t3\n]"},
+	{`{"x":1}`, "{\n\t\"x\": 1\n}"},
+	{ex1, ex1i},
+}
+
+var ex1 = `[true,false,null,"x",1,1.5,0,-5e+2]`
+
+var ex1i = `[
+	true,
+	false,
+	null,
+	"x",
+	1,
+	1.5,
+	0,
+	-5e+2
+]`
+
+func TestCompact(t *testing.T) {
+	var buf bytes.Buffer
+	for _, tt := range examples {
+		buf.Reset()
+		if err := Compact(&buf, []byte(tt.compact)); err != nil {
+			t.Errorf("Compact(%#q): %v", tt.compact, err)
+		} else if s := buf.String(); s != tt.compact {
+			t.Errorf("Compact(%#q) = %#q, want original", tt.compact, s)
+		}
+
+		buf.Reset()
+		if err := Compact(&buf, []byte(tt.indent)); err != nil {
+			t.Errorf("Compact(%#q): %v", tt.indent, err)
+			continue
+		} else if s := buf.String(); s != tt.compact {
+			t.Errorf("Compact(%#q) = %#q, want %#q", tt.indent, s, tt.compact)
+		}
+	}
+}
+
+func TestCompactSeparators(t *testing.T) {
+	// U+2028 and U+2029 should be escaped inside strings.
+	// They should not appear outside strings.
+	tests := []struct {
+		in, compact string
+	}{
+		{"{\"\u2028\": 1}", `{"\u2028":1}`},
+		{"{\"\u2029\" :2}", `{"\u2029":2}`},
+	}
+	for _, tt := range tests {
+		var buf bytes.Buffer
+		if err := Compact(&buf, []byte(tt.in)); err != nil {
+			t.Errorf("Compact(%q): %v", tt.in, err)
+		} else if s := buf.String(); s != tt.compact {
+			t.Errorf("Compact(%q) = %q, want %q", tt.in, s, tt.compact)
+		}
+	}
+}
+
+func TestIndent(t *testing.T) {
+	var buf bytes.Buffer
+	for _, tt := range examples {
+		buf.Reset()
+		if err := Indent(&buf, []byte(tt.indent), "", "\t"); err != nil {
+			t.Errorf("Indent(%#q): %v", tt.indent, err)
+		} else if s := buf.String(); s != tt.indent {
+			t.Errorf("Indent(%#q) = %#q, want original", tt.indent, s)
+		}
+
+		buf.Reset()
+		if err := Indent(&buf, []byte(tt.compact), "", "\t"); err != nil {
+			t.Errorf("Indent(%#q): %v", tt.compact, err)
+			continue
+		} else if s := buf.String(); s != tt.indent {
+			t.Errorf("Indent(%#q) = %#q, want %#q", tt.compact, s, tt.indent)
+		}
+	}
+}
+
+// Tests of a large random structure.
+
+func TestCompactBig(t *testing.T) {
+	initBig()
+	var buf bytes.Buffer
+	if err := Compact(&buf, jsonBig); err != nil {
+		t.Fatalf("Compact: %v", err)
+	}
+	b := buf.Bytes()
+	if !bytes.Equal(b, jsonBig) {
+		t.Error("Compact(jsonBig) != jsonBig")
+		diff(t, b, jsonBig)
+		return
+	}
+}
+
+func TestIndentBig(t *testing.T) {
+	t.Parallel()
+	initBig()
+	var buf bytes.Buffer
+	if err := Indent(&buf, jsonBig, "", "\t"); err != nil {
+		t.Fatalf("Indent1: %v", err)
+	}
+	b := buf.Bytes()
+	if len(b) == len(jsonBig) {
+		// jsonBig is compact (no unnecessary spaces);
+		// indenting should make it bigger
+		t.Fatalf("Indent(jsonBig) did not get bigger")
+	}
+
+	// should be idempotent
+	var buf1 bytes.Buffer
+	if err := Indent(&buf1, b, "", "\t"); err != nil {
+		t.Fatalf("Indent2: %v", err)
+	}
+	b1 := buf1.Bytes()
+	if !bytes.Equal(b1, b) {
+		t.Error("Indent(Indent(jsonBig)) != Indent(jsonBig)")
+		diff(t, b1, b)
+		return
+	}
+
+	// should get back to original
+	buf1.Reset()
+	if err := Compact(&buf1, b); err != nil {
+		t.Fatalf("Compact: %v", err)
+	}
+	b1 = buf1.Bytes()
+	if !bytes.Equal(b1, jsonBig) {
+		t.Error("Compact(Indent(jsonBig)) != jsonBig")
+		diff(t, b1, jsonBig)
+		return
+	}
+}
+
+type indentErrorTest struct {
+	in  string
+	err error
+}
+
+var indentErrorTests = []indentErrorTest{
+	{`{"X": "foo", "Y"}`, &SyntaxError{"invalid character '}' after object key", 17}},
+	{`{"X": "foo" "Y": "bar"}`, &SyntaxError{"invalid character '\"' after object key:value pair", 13}},
+}
+
+func TestIndentErrors(t *testing.T) {
+	for i, tt := range indentErrorTests {
+		slice := make([]uint8, 0)
+		buf := bytes.NewBuffer(slice)
+		if err := Indent(buf, []uint8(tt.in), "", ""); err != nil {
+			if !reflect.DeepEqual(err, tt.err) {
+				t.Errorf("#%d: Indent: %#v", i, err)
+				continue
+			}
+		}
+	}
+}
+
+func diff(t *testing.T, a, b []byte) {
+	for i := 0; ; i++ {
+		if i >= len(a) || i >= len(b) || a[i] != b[i] {
+			j := i - 10
+			if j < 0 {
+				j = 0
+			}
+			t.Errorf("diverge at %d: «%s» vs «%s»", i, trim(a[j:]), trim(b[j:]))
+			return
+		}
+	}
+}
+
+func trim(b []byte) []byte {
+	if len(b) > 20 {
+		return b[0:20]
+	}
+	return b
+}
+
+// Generate a random JSON object.
+
+var jsonBig []byte
+
+func initBig() {
+	n := 10000
+	if testing.Short() {
+		n = 100
+	}
+	b, err := Marshal(genValue(n))
+	if err != nil {
+		panic(err)
+	}
+	jsonBig = b
+}
+
+func genValue(n int) interface{} {
+	if n > 1 {
+		switch rand.Intn(2) {
+		case 0:
+			return genArray(n)
+		case 1:
+			return genMap(n)
+		}
+	}
+	switch rand.Intn(3) {
+	case 0:
+		return rand.Intn(2) == 0
+	case 1:
+		return rand.NormFloat64()
+	case 2:
+		return genString(30)
+	}
+	panic("unreachable")
+}
+
+func genString(stddev float64) string {
+	n := int(math.Abs(rand.NormFloat64()*stddev + stddev/2))
+	c := make([]rune, n)
+	for i := range c {
+		f := math.Abs(rand.NormFloat64()*64 + 32)
+		if f > 0x10ffff {
+			f = 0x10ffff
+		}
+		c[i] = rune(f)
+	}
+	return string(c)
+}
+
+func genArray(n int) []interface{} {
+	f := int(math.Abs(rand.NormFloat64()) * math.Min(10, float64(n/2)))
+	if f > n {
+		f = n
+	}
+	if f < 1 {
+		f = 1
+	}
+	x := make([]interface{}, f)
+	for i := range x {
+		x[i] = genValue(((i+1)*n)/f - (i*n)/f)
+	}
+	return x
+}
+
+func genMap(n int) map[string]interface{} {
+	f := int(math.Abs(rand.NormFloat64()) * math.Min(10, float64(n/2)))
+	if f > n {
+		f = n
+	}
+	if n > 0 && f == 0 {
+		f = 1
+	}
+	x := make(map[string]interface{})
+	for i := 0; i < f; i++ {
+		x[genString(10)] = genValue(((i+1)*n)/f - (i*n)/f)
+	}
+	return x
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/stream.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/stream.go
new file mode 100644
index 0000000..7d5137f
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/stream.go
@@ -0,0 +1,505 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package json
+
+import (
+	"bytes"
+	"errors"
+	"io"
+)
+
+// A Decoder reads and decodes JSON values from an input stream.
+type Decoder struct {
+	r       io.Reader
+	buf     []byte
+	d       decodeState
+	scanp   int   // start of unread data in buf
+	scanned int64 // amount of data already scanned
+	scan    scanner
+	err     error
+
+	tokenState int
+	tokenStack []int
+}
+
+// NewDecoder returns a new decoder that reads from r.
+//
+// The decoder introduces its own buffering and may
+// read data from r beyond the JSON values requested.
+func NewDecoder(r io.Reader) *Decoder {
+	return &Decoder{r: r}
+}
+
+// UseNumber causes the Decoder to unmarshal a number into an interface{} as a
+// Number instead of as a float64.
+func (dec *Decoder) UseNumber() { dec.d.useNumber = true }
+
+// DisallowUnknownFields causes the Decoder to return an error when the destination
+// is a struct and the input contains object keys which do not match any
+// non-ignored, exported fields in the destination.
+func (dec *Decoder) DisallowUnknownFields() { dec.d.disallowUnknownFields = true }
+
+// Decode reads the next JSON-encoded value from its
+// input and stores it in the value pointed to by v.
+//
+// See the documentation for Unmarshal for details about
+// the conversion of JSON into a Go value.
+func (dec *Decoder) Decode(v interface{}) error {
+	if dec.err != nil {
+		return dec.err
+	}
+
+	if err := dec.tokenPrepareForDecode(); err != nil {
+		return err
+	}
+
+	if !dec.tokenValueAllowed() {
+		return &SyntaxError{msg: "not at beginning of value", Offset: dec.offset()}
+	}
+
+	// Read whole value into buffer.
+	n, err := dec.readValue()
+	if err != nil {
+		return err
+	}
+	dec.d.init(dec.buf[dec.scanp : dec.scanp+n])
+	dec.scanp += n
+
+	// Don't save err from unmarshal into dec.err:
+	// the connection is still usable since we read a complete JSON
+	// object from it before the error happened.
+	err = dec.d.unmarshal(v)
+
+	// fixup token streaming state
+	dec.tokenValueEnd()
+
+	return err
+}
+
+// Buffered returns a reader of the data remaining in the Decoder's
+// buffer. The reader is valid until the next call to Decode.
+func (dec *Decoder) Buffered() io.Reader {
+	return bytes.NewReader(dec.buf[dec.scanp:])
+}
+
+// readValue reads a JSON value into dec.buf.
+// It returns the length of the encoding.
+func (dec *Decoder) readValue() (int, error) {
+	dec.scan.reset()
+
+	scanp := dec.scanp
+	var err error
+Input:
+	for {
+		// Look in the buffer for a new value.
+		for i, c := range dec.buf[scanp:] {
+			dec.scan.bytes++
+			switch dec.scan.step(&dec.scan, c) {
+			case scanEnd:
+				scanp += i
+				break Input
+			case scanEndObject, scanEndArray:
+				// scanEnd is delayed one byte.
+				// We might block trying to get that byte from src,
+				// so instead invent a space byte.
+				if stateEndValue(&dec.scan, ' ') == scanEnd {
+					scanp += i + 1
+					break Input
+				}
+			case scanError:
+				dec.err = dec.scan.err
+				return 0, dec.scan.err
+			}
+		}
+		scanp = len(dec.buf)
+
+		// Did the last read have an error?
+		// Delayed until now to allow buffer scan.
+		if err != nil {
+			if err == io.EOF {
+				if dec.scan.step(&dec.scan, ' ') == scanEnd {
+					break Input
+				}
+				if nonSpace(dec.buf) {
+					err = io.ErrUnexpectedEOF
+				}
+			}
+			dec.err = err
+			return 0, err
+		}
+
+		n := scanp - dec.scanp
+		err = dec.refill()
+		scanp = dec.scanp + n
+	}
+	return scanp - dec.scanp, nil
+}
+
+func (dec *Decoder) refill() error {
+	// Make room to read more into the buffer.
+	// First slide down data already consumed.
+	if dec.scanp > 0 {
+		dec.scanned += int64(dec.scanp)
+		n := copy(dec.buf, dec.buf[dec.scanp:])
+		dec.buf = dec.buf[:n]
+		dec.scanp = 0
+	}
+
+	// Grow buffer if not large enough.
+	const minRead = 512
+	if cap(dec.buf)-len(dec.buf) < minRead {
+		newBuf := make([]byte, len(dec.buf), 2*cap(dec.buf)+minRead)
+		copy(newBuf, dec.buf)
+		dec.buf = newBuf
+	}
+
+	// Read. Delay error for next iteration (after scan).
+	n, err := dec.r.Read(dec.buf[len(dec.buf):cap(dec.buf)])
+	dec.buf = dec.buf[0 : len(dec.buf)+n]
+
+	return err
+}
+
+func nonSpace(b []byte) bool {
+	for _, c := range b {
+		if !isSpace(c) {
+			return true
+		}
+	}
+	return false
+}
+
+// An Encoder writes JSON values to an output stream.
+type Encoder struct {
+	w          io.Writer
+	err        error
+	escapeHTML bool
+
+	indentBuf    *bytes.Buffer
+	indentPrefix string
+	indentValue  string
+}
+
+// NewEncoder returns a new encoder that writes to w.
+func NewEncoder(w io.Writer) *Encoder {
+	return &Encoder{w: w, escapeHTML: true}
+}
+
+// Encode writes the JSON encoding of v to the stream,
+// followed by a newline character.
+//
+// See the documentation for Marshal for details about the
+// conversion of Go values to JSON.
+func (enc *Encoder) Encode(v interface{}) error {
+	if enc.err != nil {
+		return enc.err
+	}
+	e := newEncodeState()
+	err := e.marshal(v, encOpts{escapeHTML: enc.escapeHTML})
+	if err != nil {
+		return err
+	}
+
+	// Terminate each value with a newline.
+	// This makes the output look a little nicer
+	// when debugging, and some kind of space
+	// is required if the encoded value was a number,
+	// so that the reader knows there aren't more
+	// digits coming.
+	e.WriteByte('\n')
+
+	b := e.Bytes()
+	if enc.indentPrefix != "" || enc.indentValue != "" {
+		if enc.indentBuf == nil {
+			enc.indentBuf = new(bytes.Buffer)
+		}
+		enc.indentBuf.Reset()
+		err = Indent(enc.indentBuf, b, enc.indentPrefix, enc.indentValue)
+		if err != nil {
+			return err
+		}
+		b = enc.indentBuf.Bytes()
+	}
+	if _, err = enc.w.Write(b); err != nil {
+		enc.err = err
+	}
+	encodeStatePool.Put(e)
+	return err
+}
+
+// SetIndent instructs the encoder to format each subsequent encoded
+// value as if indented by the package-level function Indent(dst, src, prefix, indent).
+// Calling SetIndent("", "") disables indentation.
+func (enc *Encoder) SetIndent(prefix, indent string) {
+	enc.indentPrefix = prefix
+	enc.indentValue = indent
+}
+
+// SetEscapeHTML specifies whether problematic HTML characters
+// should be escaped inside JSON quoted strings.
+// The default behavior is to escape &, <, and > to \u0026, \u003c, and \u003e
+// to avoid certain safety problems that can arise when embedding JSON in HTML.
+//
+// In non-HTML settings where the escaping interferes with the readability
+// of the output, SetEscapeHTML(false) disables this behavior.
+func (enc *Encoder) SetEscapeHTML(on bool) {
+	enc.escapeHTML = on
+}
+
+// RawMessage is a raw encoded JSON value.
+// It implements Marshaler and Unmarshaler and can
+// be used to delay JSON decoding or precompute a JSON encoding.
+type RawMessage []byte
+
+// MarshalJSON returns m as the JSON encoding of m.
+func (m RawMessage) MarshalJSON() ([]byte, error) {
+	if m == nil {
+		return []byte("null"), nil
+	}
+	return m, nil
+}
+
+// UnmarshalJSON sets *m to a copy of data.
+func (m *RawMessage) UnmarshalJSON(data []byte) error {
+	if m == nil {
+		return errors.New("json.RawMessage: UnmarshalJSON on nil pointer")
+	}
+	*m = append((*m)[0:0], data...)
+	return nil
+}
+
+var _ Marshaler = (*RawMessage)(nil)
+var _ Unmarshaler = (*RawMessage)(nil)
+
+// A Token holds a value of one of these types:
+//
+//	Delim, for the four JSON delimiters [ ] { }
+//	bool, for JSON booleans
+//	float64, for JSON numbers
+//	Number, for JSON numbers
+//	string, for JSON string literals
+//	nil, for JSON null
+//
+type Token interface{}
+
+const (
+	tokenTopValue = iota
+	tokenArrayStart
+	tokenArrayValue
+	tokenArrayComma
+	tokenObjectStart
+	tokenObjectKey
+	tokenObjectColon
+	tokenObjectValue
+	tokenObjectComma
+)
+
+// advance tokenstate from a separator state to a value state
+func (dec *Decoder) tokenPrepareForDecode() error {
+	// Note: Not calling peek before switch, to avoid
+	// putting peek into the standard Decode path.
+	// peek is only called when using the Token API.
+	switch dec.tokenState {
+	case tokenArrayComma:
+		c, err := dec.peek()
+		if err != nil {
+			return err
+		}
+		if c != ',' {
+			return &SyntaxError{"expected comma after array element", dec.offset()}
+		}
+		dec.scanp++
+		dec.tokenState = tokenArrayValue
+	case tokenObjectColon:
+		c, err := dec.peek()
+		if err != nil {
+			return err
+		}
+		if c != ':' {
+			return &SyntaxError{"expected colon after object key", dec.offset()}
+		}
+		dec.scanp++
+		dec.tokenState = tokenObjectValue
+	}
+	return nil
+}
+
+func (dec *Decoder) tokenValueAllowed() bool {
+	switch dec.tokenState {
+	case tokenTopValue, tokenArrayStart, tokenArrayValue, tokenObjectValue:
+		return true
+	}
+	return false
+}
+
+func (dec *Decoder) tokenValueEnd() {
+	switch dec.tokenState {
+	case tokenArrayStart, tokenArrayValue:
+		dec.tokenState = tokenArrayComma
+	case tokenObjectValue:
+		dec.tokenState = tokenObjectComma
+	}
+}
+
+// A Delim is a JSON array or object delimiter, one of [ ] { or }.
+type Delim rune
+
+func (d Delim) String() string {
+	return string(d)
+}
+
+// Token returns the next JSON token in the input stream.
+// At the end of the input stream, Token returns nil, io.EOF.
+//
+// Token guarantees that the delimiters [ ] { } it returns are
+// properly nested and matched: if Token encounters an unexpected
+// delimiter in the input, it will return an error.
+//
+// The input stream consists of basic JSON values—bool, string,
+// number, and null—along with delimiters [ ] { } of type Delim
+// to mark the start and end of arrays and objects.
+// Commas and colons are elided.
+func (dec *Decoder) Token() (Token, error) {
+	for {
+		c, err := dec.peek()
+		if err != nil {
+			return nil, err
+		}
+		switch c {
+		case '[':
+			if !dec.tokenValueAllowed() {
+				return dec.tokenError(c)
+			}
+			dec.scanp++
+			dec.tokenStack = append(dec.tokenStack, dec.tokenState)
+			dec.tokenState = tokenArrayStart
+			return Delim('['), nil
+
+		case ']':
+			if dec.tokenState != tokenArrayStart && dec.tokenState != tokenArrayComma {
+				return dec.tokenError(c)
+			}
+			dec.scanp++
+			dec.tokenState = dec.tokenStack[len(dec.tokenStack)-1]
+			dec.tokenStack = dec.tokenStack[:len(dec.tokenStack)-1]
+			dec.tokenValueEnd()
+			return Delim(']'), nil
+
+		case '{':
+			if !dec.tokenValueAllowed() {
+				return dec.tokenError(c)
+			}
+			dec.scanp++
+			dec.tokenStack = append(dec.tokenStack, dec.tokenState)
+			dec.tokenState = tokenObjectStart
+			return Delim('{'), nil
+
+		case '}':
+			if dec.tokenState != tokenObjectStart && dec.tokenState != tokenObjectComma {
+				return dec.tokenError(c)
+			}
+			dec.scanp++
+			dec.tokenState = dec.tokenStack[len(dec.tokenStack)-1]
+			dec.tokenStack = dec.tokenStack[:len(dec.tokenStack)-1]
+			dec.tokenValueEnd()
+			return Delim('}'), nil
+
+		case ':':
+			if dec.tokenState != tokenObjectColon {
+				return dec.tokenError(c)
+			}
+			dec.scanp++
+			dec.tokenState = tokenObjectValue
+			continue
+
+		case ',':
+			if dec.tokenState == tokenArrayComma {
+				dec.scanp++
+				dec.tokenState = tokenArrayValue
+				continue
+			}
+			if dec.tokenState == tokenObjectComma {
+				dec.scanp++
+				dec.tokenState = tokenObjectKey
+				continue
+			}
+			return dec.tokenError(c)
+
+		case '"':
+			if dec.tokenState == tokenObjectStart || dec.tokenState == tokenObjectKey {
+				var x string
+				old := dec.tokenState
+				dec.tokenState = tokenTopValue
+				err := dec.Decode(&x)
+				dec.tokenState = old
+				if err != nil {
+					return nil, err
+				}
+				dec.tokenState = tokenObjectColon
+				return x, nil
+			}
+			fallthrough
+
+		default:
+			if !dec.tokenValueAllowed() {
+				return dec.tokenError(c)
+			}
+			var x interface{}
+			if err := dec.Decode(&x); err != nil {
+				return nil, err
+			}
+			return x, nil
+		}
+	}
+}
+
+func (dec *Decoder) tokenError(c byte) (Token, error) {
+	var context string
+	switch dec.tokenState {
+	case tokenTopValue:
+		context = " looking for beginning of value"
+	case tokenArrayStart, tokenArrayValue, tokenObjectValue:
+		context = " looking for beginning of value"
+	case tokenArrayComma:
+		context = " after array element"
+	case tokenObjectKey:
+		context = " looking for beginning of object key string"
+	case tokenObjectColon:
+		context = " after object key"
+	case tokenObjectComma:
+		context = " after object key:value pair"
+	}
+	return nil, &SyntaxError{"invalid character " + quoteChar(c) + context, dec.offset()}
+}
+
+// More reports whether there is another element in the
+// current array or object being parsed.
+func (dec *Decoder) More() bool {
+	c, err := dec.peek()
+	return err == nil && c != ']' && c != '}'
+}
+
+func (dec *Decoder) peek() (byte, error) {
+	var err error
+	for {
+		for i := dec.scanp; i < len(dec.buf); i++ {
+			c := dec.buf[i]
+			if isSpace(c) {
+				continue
+			}
+			dec.scanp = i
+			return c, nil
+		}
+		// buffer has been scanned, now report any error
+		if err != nil {
+			return 0, err
+		}
+		err = dec.refill()
+	}
+}
+
+func (dec *Decoder) offset() int64 {
+	return dec.scanned + int64(dec.scanp)
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/stream_test.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/stream_test.go
new file mode 100644
index 0000000..97eee72
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/stream_test.go
@@ -0,0 +1,438 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package json
+
+import (
+	"bytes"
+	"io"
+	"log"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+// Test values for the stream test.
+// One of each JSON kind.
+var streamTest = []interface{}{
+	0.1,
+	"hello",
+	nil,
+	true,
+	false,
+	[]interface{}{"a", "b", "c"},
+	map[string]interface{}{"K": "Kelvin", "ß": "long s"},
+	3.14, // another value to make sure something can follow map
+}
+
+var streamEncoded = `0.1
+"hello"
+null
+true
+false
+["a","b","c"]
+{"ß":"long s","K":"Kelvin"}
+3.14
+`
+
+func TestEncoder(t *testing.T) {
+	for i := 0; i <= len(streamTest); i++ {
+		var buf bytes.Buffer
+		enc := NewEncoder(&buf)
+		// Check that enc.SetIndent("", "") turns off indentation.
+		enc.SetIndent(">", ".")
+		enc.SetIndent("", "")
+		for j, v := range streamTest[0:i] {
+			if err := enc.Encode(v); err != nil {
+				t.Fatalf("encode #%d: %v", j, err)
+			}
+		}
+		if have, want := buf.String(), nlines(streamEncoded, i); have != want {
+			t.Errorf("encoding %d items: mismatch", i)
+			diff(t, []byte(have), []byte(want))
+			break
+		}
+	}
+}
+
+var streamEncodedIndent = `0.1
+"hello"
+null
+true
+false
+[
+>."a",
+>."b",
+>."c"
+>]
+{
+>."ß": "long s",
+>."K": "Kelvin"
+>}
+3.14
+`
+
+func TestEncoderIndent(t *testing.T) {
+	var buf bytes.Buffer
+	enc := NewEncoder(&buf)
+	enc.SetIndent(">", ".")
+	for _, v := range streamTest {
+		enc.Encode(v)
+	}
+	if have, want := buf.String(), streamEncodedIndent; have != want {
+		t.Error("indented encoding mismatch")
+		diff(t, []byte(have), []byte(want))
+	}
+}
+
+func TestEncoderSetEscapeHTML(t *testing.T) {
+	var c C
+	var ct CText
+	var tagStruct struct {
+		Valid   int `json:"<>&#! "`
+		Invalid int `json:"\\"`
+	}
+	for _, tt := range []struct {
+		name       string
+		v          interface{}
+		wantEscape string
+		want       string
+	}{
+		{"c", c, `"\u003c\u0026\u003e"`, `"<&>"`},
+		{"ct", ct, `"\"\u003c\u0026\u003e\""`, `"\"<&>\""`},
+		{`"<&>"`, "<&>", `"\u003c\u0026\u003e"`, `"<&>"`},
+		{
+			"tagStruct", tagStruct,
+			`{"\u003c\u003e\u0026#! ":0,"Invalid":0}`,
+			`{"<>&#! ":0,"Invalid":0}`,
+		},
+	} {
+		var buf bytes.Buffer
+		enc := NewEncoder(&buf)
+		if err := enc.Encode(tt.v); err != nil {
+			t.Fatalf("Encode(%s): %s", tt.name, err)
+		}
+		if got := strings.TrimSpace(buf.String()); got != tt.wantEscape {
+			t.Errorf("Encode(%s) = %#q, want %#q", tt.name, got, tt.wantEscape)
+		}
+		buf.Reset()
+		enc.SetEscapeHTML(false)
+		if err := enc.Encode(tt.v); err != nil {
+			t.Fatalf("SetEscapeHTML(false) Encode(%s): %s", tt.name, err)
+		}
+		if got := strings.TrimSpace(buf.String()); got != tt.want {
+			t.Errorf("SetEscapeHTML(false) Encode(%s) = %#q, want %#q",
+				tt.name, got, tt.want)
+		}
+	}
+}
+
+func TestDecoder(t *testing.T) {
+	for i := 0; i <= len(streamTest); i++ {
+		// Use stream without newlines as input,
+		// just to stress the decoder even more.
+		// Our test input does not include back-to-back numbers.
+		// Otherwise stripping the newlines would
+		// merge two adjacent JSON values.
+		var buf bytes.Buffer
+		for _, c := range nlines(streamEncoded, i) {
+			if c != '\n' {
+				buf.WriteRune(c)
+			}
+		}
+		out := make([]interface{}, i)
+		dec := NewDecoder(&buf)
+		for j := range out {
+			if err := dec.Decode(&out[j]); err != nil {
+				t.Fatalf("decode #%d/%d: %v", j, i, err)
+			}
+		}
+		if !reflect.DeepEqual(out, streamTest[0:i]) {
+			t.Errorf("decoding %d items: mismatch", i)
+			for j := range out {
+				if !reflect.DeepEqual(out[j], streamTest[j]) {
+					t.Errorf("#%d: have %v want %v", j, out[j], streamTest[j])
+				}
+			}
+			break
+		}
+	}
+}
+
+func TestDecoderBuffered(t *testing.T) {
+	r := strings.NewReader(`{"Name": "Gopher"} extra `)
+	var m struct {
+		Name string
+	}
+	d := NewDecoder(r)
+	err := d.Decode(&m)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if m.Name != "Gopher" {
+		t.Errorf("Name = %q; want Gopher", m.Name)
+	}
+	rest, err := io.ReadAll(d.Buffered())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if g, w := string(rest), " extra "; g != w {
+		t.Errorf("Remaining = %q; want %q", g, w)
+	}
+}
+
+func nlines(s string, n int) string {
+	if n <= 0 {
+		return ""
+	}
+	for i, c := range s {
+		if c == '\n' {
+			if n--; n == 0 {
+				return s[0 : i+1]
+			}
+		}
+	}
+	return s
+}
+
+func TestRawMessage(t *testing.T) {
+	var data struct {
+		X  float64
+		Id RawMessage
+		Y  float32
+	}
+	const raw = `["\u0056",null]`
+	const msg = `{"X":0.1,"Id":["\u0056",null],"Y":0.2}`
+	err := Unmarshal([]byte(msg), &data)
+	if err != nil {
+		t.Fatalf("Unmarshal: %v", err)
+	}
+	if string([]byte(data.Id)) != raw {
+		t.Fatalf("Raw mismatch: have %#q want %#q", []byte(data.Id), raw)
+	}
+	b, err := Marshal(&data)
+	if err != nil {
+		t.Fatalf("Marshal: %v", err)
+	}
+	if string(b) != msg {
+		t.Fatalf("Marshal: have %#q want %#q", b, msg)
+	}
+}
+
+func TestNullRawMessage(t *testing.T) {
+	var data struct {
+		X     float64
+		Id    RawMessage
+		IdPtr *RawMessage
+		Y     float32
+	}
+	const msg = `{"X":0.1,"Id":null,"IdPtr":null,"Y":0.2}`
+	err := Unmarshal([]byte(msg), &data)
+	if err != nil {
+		t.Fatalf("Unmarshal: %v", err)
+	}
+	if want, got := "null", string(data.Id); want != got {
+		t.Fatalf("Raw mismatch: have %q, want %q", got, want)
+	}
+	if data.IdPtr != nil {
+		t.Fatalf("Raw pointer mismatch: have non-nil, want nil")
+	}
+	b, err := Marshal(&data)
+	if err != nil {
+		t.Fatalf("Marshal: %v", err)
+	}
+	if string(b) != msg {
+		t.Fatalf("Marshal: have %#q want %#q", b, msg)
+	}
+}
+
+var blockingTests = []string{
+	`{"x": 1}`,
+	`[1, 2, 3]`,
+}
+
+func TestBlocking(t *testing.T) {
+	for _, enc := range blockingTests {
+		r, w := net.Pipe()
+		go w.Write([]byte(enc))
+		var val interface{}
+
+		// If Decode reads beyond what w.Write writes above,
+		// it will block, and the test will deadlock.
+		if err := NewDecoder(r).Decode(&val); err != nil {
+			t.Errorf("decoding %s: %v", enc, err)
+		}
+		r.Close()
+		w.Close()
+	}
+}
+
+func BenchmarkEncoderEncode(b *testing.B) {
+	b.ReportAllocs()
+	type T struct {
+		X, Y string
+	}
+	v := &T{"foo", "bar"}
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			if err := NewEncoder(io.Discard).Encode(v); err != nil {
+				b.Fatal(err)
+			}
+		}
+	})
+}
+
+type tokenStreamCase struct {
+	json      string
+	expTokens []interface{}
+}
+
+type decodeThis struct {
+	v interface{}
+}
+
+var tokenStreamCases []tokenStreamCase = []tokenStreamCase{
+	// streaming token cases
+	{json: `10`, expTokens: []interface{}{float64(10)}},
+	{json: ` [10] `, expTokens: []interface{}{
+		Delim('['), float64(10), Delim(']')}},
+	{json: ` [false,10,"b"] `, expTokens: []interface{}{
+		Delim('['), false, float64(10), "b", Delim(']')}},
+	{json: `{ "a": 1 }`, expTokens: []interface{}{
+		Delim('{'), "a", float64(1), Delim('}')}},
+	{json: `{"a": 1, "b":"3"}`, expTokens: []interface{}{
+		Delim('{'), "a", float64(1), "b", "3", Delim('}')}},
+	{json: ` [{"a": 1},{"a": 2}] `, expTokens: []interface{}{
+		Delim('['),
+		Delim('{'), "a", float64(1), Delim('}'),
+		Delim('{'), "a", float64(2), Delim('}'),
+		Delim(']')}},
+	{json: `{"obj": {"a": 1}}`, expTokens: []interface{}{
+		Delim('{'), "obj", Delim('{'), "a", float64(1), Delim('}'),
+		Delim('}')}},
+	{json: `{"obj": [{"a": 1}]}`, expTokens: []interface{}{
+		Delim('{'), "obj", Delim('['),
+		Delim('{'), "a", float64(1), Delim('}'),
+		Delim(']'), Delim('}')}},
+
+	// streaming tokens with intermittent Decode()
+	{json: `{ "a": 1 }`, expTokens: []interface{}{
+		Delim('{'), "a",
+		decodeThis{float64(1)},
+		Delim('}')}},
+	{json: ` [ { "a" : 1 } ] `, expTokens: []interface{}{
+		Delim('['),
+		decodeThis{map[string]interface{}{"a": float64(1)}},
+		Delim(']')}},
+	{json: ` [{"a": 1},{"a": 2}] `, expTokens: []interface{}{
+		Delim('['),
+		decodeThis{map[string]interface{}{"a": float64(1)}},
+		decodeThis{map[string]interface{}{"a": float64(2)}},
+		Delim(']')}},
+	{json: `{ "obj" : [ { "a" : 1 } ] }`, expTokens: []interface{}{
+		Delim('{'), "obj", Delim('['),
+		decodeThis{map[string]interface{}{"a": float64(1)}},
+		Delim(']'), Delim('}')}},
+
+	{json: `{"obj": {"a": 1}}`, expTokens: []interface{}{
+		Delim('{'), "obj",
+		decodeThis{map[string]interface{}{"a": float64(1)}},
+		Delim('}')}},
+	{json: `{"obj": [{"a": 1}]}`, expTokens: []interface{}{
+		Delim('{'), "obj",
+		decodeThis{[]interface{}{
+			map[string]interface{}{"a": float64(1)},
+		}},
+		Delim('}')}},
+	{json: ` [{"a": 1} {"a": 2}] `, expTokens: []interface{}{
+		Delim('['),
+		decodeThis{map[string]interface{}{"a": float64(1)}},
+		decodeThis{&SyntaxError{"expected comma after array element", 11}},
+	}},
+	{json: `{ "` + strings.Repeat("a", 513) + `" 1 }`, expTokens: []interface{}{
+		Delim('{'), strings.Repeat("a", 513),
+		decodeThis{&SyntaxError{"expected colon after object key", 518}},
+	}},
+	{json: `{ "\a" }`, expTokens: []interface{}{
+		Delim('{'),
+		&SyntaxError{"invalid character 'a' in string escape code", 3},
+	}},
+	{json: ` \a`, expTokens: []interface{}{
+		&SyntaxError{"invalid character '\\\\' looking for beginning of value", 1},
+	}},
+}
+
+func TestDecodeInStream(t *testing.T) {
+
+	for ci, tcase := range tokenStreamCases {
+
+		dec := NewDecoder(strings.NewReader(tcase.json))
+		for i, etk := range tcase.expTokens {
+
+			var tk interface{}
+			var err error
+
+			if dt, ok := etk.(decodeThis); ok {
+				etk = dt.v
+				err = dec.Decode(&tk)
+			} else {
+				tk, err = dec.Token()
+			}
+			if experr, ok := etk.(error); ok {
+				if err == nil || !reflect.DeepEqual(err, experr) {
+					t.Errorf("case %v: Expected error %#v in %q, but was %#v", ci, experr, tcase.json, err)
+				}
+				break
+			} else if err == io.EOF {
+				t.Errorf("case %v: Unexpected EOF in %q", ci, tcase.json)
+				break
+			} else if err != nil {
+				t.Errorf("case %v: Unexpected error '%#v' in %q", ci, err, tcase.json)
+				break
+			}
+			if !reflect.DeepEqual(tk, etk) {
+				t.Errorf(`case %v: %q @ %v expected %T(%v) was %T(%v)`, ci, tcase.json, i, etk, etk, tk, tk)
+				break
+			}
+		}
+	}
+
+}
+
+// Test from golang.org/issue/11893
+func TestHTTPDecoding(t *testing.T) {
+	const raw = `{ "foo": "bar" }`
+
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Write([]byte(raw))
+	}))
+	defer ts.Close()
+	res, err := http.Get(ts.URL)
+	if err != nil {
+		log.Fatalf("GET failed: %v", err)
+	}
+	defer res.Body.Close()
+
+	foo := struct {
+		Foo string
+	}{}
+
+	d := NewDecoder(res.Body)
+	err = d.Decode(&foo)
+	if err != nil {
+		t.Fatalf("Decode: %v", err)
+	}
+	if foo.Foo != "bar" {
+		t.Errorf("decoded %q; want \"bar\"", foo.Foo)
+	}
+
+	// make sure we get the EOF the second time
+	err = d.Decode(&foo)
+	if err != io.EOF {
+		t.Errorf("err = %v; want io.EOF", err)
+	}
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/tables.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/tables.go
new file mode 100644
index 0000000..10acdc1
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/tables.go
@@ -0,0 +1,218 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package json
+
+import "unicode/utf8"
+
+// safeSet holds the value true if the ASCII character with the given array
+// position can be represented inside a JSON string without any further
+// escaping.
+//
+// All values are true except for the ASCII control characters (0-31), the
+// double quote ("), and the backslash character ("\").
+var safeSet = [utf8.RuneSelf]bool{
+	' ':      true,
+	'!':      true,
+	'"':      false,
+	'#':      true,
+	'$':      true,
+	'%':      true,
+	'&':      true,
+	'\'':     true,
+	'(':      true,
+	')':      true,
+	'*':      true,
+	'+':      true,
+	',':      true,
+	'-':      true,
+	'.':      true,
+	'/':      true,
+	'0':      true,
+	'1':      true,
+	'2':      true,
+	'3':      true,
+	'4':      true,
+	'5':      true,
+	'6':      true,
+	'7':      true,
+	'8':      true,
+	'9':      true,
+	':':      true,
+	';':      true,
+	'<':      true,
+	'=':      true,
+	'>':      true,
+	'?':      true,
+	'@':      true,
+	'A':      true,
+	'B':      true,
+	'C':      true,
+	'D':      true,
+	'E':      true,
+	'F':      true,
+	'G':      true,
+	'H':      true,
+	'I':      true,
+	'J':      true,
+	'K':      true,
+	'L':      true,
+	'M':      true,
+	'N':      true,
+	'O':      true,
+	'P':      true,
+	'Q':      true,
+	'R':      true,
+	'S':      true,
+	'T':      true,
+	'U':      true,
+	'V':      true,
+	'W':      true,
+	'X':      true,
+	'Y':      true,
+	'Z':      true,
+	'[':      true,
+	'\\':     false,
+	']':      true,
+	'^':      true,
+	'_':      true,
+	'`':      true,
+	'a':      true,
+	'b':      true,
+	'c':      true,
+	'd':      true,
+	'e':      true,
+	'f':      true,
+	'g':      true,
+	'h':      true,
+	'i':      true,
+	'j':      true,
+	'k':      true,
+	'l':      true,
+	'm':      true,
+	'n':      true,
+	'o':      true,
+	'p':      true,
+	'q':      true,
+	'r':      true,
+	's':      true,
+	't':      true,
+	'u':      true,
+	'v':      true,
+	'w':      true,
+	'x':      true,
+	'y':      true,
+	'z':      true,
+	'{':      true,
+	'|':      true,
+	'}':      true,
+	'~':      true,
+	'\u007f': true,
+}
+
+// htmlSafeSet holds the value true if the ASCII character with the given
+// array position can be safely represented inside a JSON string, embedded
+// inside of HTML <script> tags, without any additional escaping.
+//
+// All values are true except for the ASCII control characters (0-31), the
+// double quote ("), the backslash character ("\"), HTML opening and closing
+// tags ("<" and ">"), and the ampersand ("&").
+var htmlSafeSet = [utf8.RuneSelf]bool{
+	' ':      true,
+	'!':      true,
+	'"':      false,
+	'#':      true,
+	'$':      true,
+	'%':      true,
+	'&':      false,
+	'\'':     true,
+	'(':      true,
+	')':      true,
+	'*':      true,
+	'+':      true,
+	',':      true,
+	'-':      true,
+	'.':      true,
+	'/':      true,
+	'0':      true,
+	'1':      true,
+	'2':      true,
+	'3':      true,
+	'4':      true,
+	'5':      true,
+	'6':      true,
+	'7':      true,
+	'8':      true,
+	'9':      true,
+	':':      true,
+	';':      true,
+	'<':      false,
+	'=':      true,
+	'>':      false,
+	'?':      true,
+	'@':      true,
+	'A':      true,
+	'B':      true,
+	'C':      true,
+	'D':      true,
+	'E':      true,
+	'F':      true,
+	'G':      true,
+	'H':      true,
+	'I':      true,
+	'J':      true,
+	'K':      true,
+	'L':      true,
+	'M':      true,
+	'N':      true,
+	'O':      true,
+	'P':      true,
+	'Q':      true,
+	'R':      true,
+	'S':      true,
+	'T':      true,
+	'U':      true,
+	'V':      true,
+	'W':      true,
+	'X':      true,
+	'Y':      true,
+	'Z':      true,
+	'[':      true,
+	'\\':     false,
+	']':      true,
+	'^':      true,
+	'_':      true,
+	'`':      true,
+	'a':      true,
+	'b':      true,
+	'c':      true,
+	'd':      true,
+	'e':      true,
+	'f':      true,
+	'g':      true,
+	'h':      true,
+	'i':      true,
+	'j':      true,
+	'k':      true,
+	'l':      true,
+	'm':      true,
+	'n':      true,
+	'o':      true,
+	'p':      true,
+	'q':      true,
+	'r':      true,
+	's':      true,
+	't':      true,
+	'u':      true,
+	'v':      true,
+	'w':      true,
+	'x':      true,
+	'y':      true,
+	'z':      true,
+	'{':      true,
+	'|':      true,
+	'}':      true,
+	'~':      true,
+	'\u007f': true,
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/tagkey_test.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/tagkey_test.go
new file mode 100644
index 0000000..f77c49c
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/tagkey_test.go
@@ -0,0 +1,120 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package json
+
+import (
+	"testing"
+)
+
+type basicLatin2xTag struct {
+	V string `json:"$%-/"`
+}
+
+type basicLatin3xTag struct {
+	V string `json:"0123456789"`
+}
+
+type basicLatin4xTag struct {
+	V string `json:"ABCDEFGHIJKLMO"`
+}
+
+type basicLatin5xTag struct {
+	V string `json:"PQRSTUVWXYZ_"`
+}
+
+type basicLatin6xTag struct {
+	V string `json:"abcdefghijklmno"`
+}
+
+type basicLatin7xTag struct {
+	V string `json:"pqrstuvwxyz"`
+}
+
+type miscPlaneTag struct {
+	V string `json:"色は匂へど"`
+}
+
+type percentSlashTag struct {
+	V string `json:"text/html%"` // https://golang.org/issue/2718
+}
+
+type punctuationTag struct {
+	V string `json:"!#$%&()*+-./:<=>?@[]^_{|}~"` // https://golang.org/issue/3546
+}
+
+type dashTag struct {
+	V string `json:"-,"`
+}
+
+type emptyTag struct {
+	W string
+}
+
+type misnamedTag struct {
+	X string `jsom:"Misnamed"`
+}
+
+type badFormatTag struct {
+	Y string `:"BadFormat"`
+}
+
+type badCodeTag struct {
+	Z string `json:" !\"#&'()*+,."`
+}
+
+type spaceTag struct {
+	Q string `json:"With space"`
+}
+
+type unicodeTag struct {
+	W string `json:"Ελλάδα"`
+}
+
+var structTagObjectKeyTests = []struct {
+	raw   interface{}
+	value string
+	key   string
+}{
+	{basicLatin2xTag{"2x"}, "2x", "$%-/"},
+	{basicLatin3xTag{"3x"}, "3x", "0123456789"},
+	{basicLatin4xTag{"4x"}, "4x", "ABCDEFGHIJKLMO"},
+	{basicLatin5xTag{"5x"}, "5x", "PQRSTUVWXYZ_"},
+	{basicLatin6xTag{"6x"}, "6x", "abcdefghijklmno"},
+	{basicLatin7xTag{"7x"}, "7x", "pqrstuvwxyz"},
+	{miscPlaneTag{"いろはにほへと"}, "いろはにほへと", "色は匂へど"},
+	{dashTag{"foo"}, "foo", "-"},
+	{emptyTag{"Pour Moi"}, "Pour Moi", "W"},
+	{misnamedTag{"Animal Kingdom"}, "Animal Kingdom", "X"},
+	{badFormatTag{"Orfevre"}, "Orfevre", "Y"},
+	{badCodeTag{"Reliable Man"}, "Reliable Man", "Z"},
+	{percentSlashTag{"brut"}, "brut", "text/html%"},
+	{punctuationTag{"Union Rags"}, "Union Rags", "!#$%&()*+-./:<=>?@[]^_{|}~"},
+	{spaceTag{"Perreddu"}, "Perreddu", "With space"},
+	{unicodeTag{"Loukanikos"}, "Loukanikos", "Ελλάδα"},
+}
+
+func TestStructTagObjectKey(t *testing.T) {
+	for _, tt := range structTagObjectKeyTests {
+		b, err := Marshal(tt.raw)
+		if err != nil {
+			t.Fatalf("Marshal(%#q) failed: %v", tt.raw, err)
+		}
+		var f interface{}
+		err = Unmarshal(b, &f)
+		if err != nil {
+			t.Fatalf("Unmarshal(%#q) failed: %v", b, err)
+		}
+		for i, v := range f.(map[string]interface{}) {
+			switch i {
+			case tt.key:
+				if s, ok := v.(string); !ok || s != tt.value {
+					t.Fatalf("Unexpected value: %#q, want %v", s, tt.value)
+				}
+			default:
+				t.Fatalf("Unexpected key: %#q, from %#q", i, b)
+			}
+		}
+	}
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/tags.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/tags.go
new file mode 100644
index 0000000..c38fd51
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/tags.go
@@ -0,0 +1,44 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package json
+
+import (
+	"strings"
+)
+
+// tagOptions is the string following a comma in a struct field's "json"
+// tag, or the empty string. It does not include the leading comma.
+type tagOptions string
+
+// parseTag splits a struct field's json tag into its name and
+// comma-separated options.
+func parseTag(tag string) (string, tagOptions) {
+	if idx := strings.Index(tag, ","); idx != -1 {
+		return tag[:idx], tagOptions(tag[idx+1:])
+	}
+	return tag, tagOptions("")
+}
+
+// Contains reports whether a comma-separated list of options
+// contains a particular substr flag. substr must be surrounded by a
+// string boundary or commas.
+func (o tagOptions) Contains(optionName string) bool {
+	if len(o) == 0 {
+		return false
+	}
+	s := string(o)
+	for s != "" {
+		var next string
+		i := strings.Index(s, ",")
+		if i >= 0 {
+			s, next = s[:i], s[i+1:]
+		}
+		if s == optionName {
+			return true
+		}
+		s = next
+	}
+	return false
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/encoding/json/tags_test.go b/internal/stdlib/testdata/v1.12.5/src/encoding/json/tags_test.go
new file mode 100644
index 0000000..8ba8ddd
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/encoding/json/tags_test.go
@@ -0,0 +1,28 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package json
+
+import (
+	"testing"
+)
+
+func TestTagParsing(t *testing.T) {
+	name, opts := parseTag("field,foobar,foo")
+	if name != "field" {
+		t.Fatalf("name = %q, want field", name)
+	}
+	for _, tt := range []struct {
+		opt  string
+		want bool
+	}{
+		{"foobar", true},
+		{"foo", true},
+		{"bar", false},
+	} {
+		if opts.Contains(tt.opt) != tt.want {
+			t.Errorf("Contains(%q) = %v", tt.opt, !tt.want)
+		}
+	}
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/errors/errors.go b/internal/stdlib/testdata/v1.12.5/src/errors/errors.go
new file mode 100644
index 0000000..b8a4692
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/errors/errors.go
@@ -0,0 +1,20 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package errors implements functions to manipulate errors.
+package errors
+
+// New returns an error that formats as the given text.
+func New(text string) error {
+	return &errorString{text}
+}
+
+// errorString is a trivial implementation of error.
+type errorString struct {
+	s string
+}
+
+func (e *errorString) Error() string {
+	return e.s
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/errors/errors_test.go b/internal/stdlib/testdata/v1.12.5/src/errors/errors_test.go
new file mode 100644
index 0000000..cf4df90
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/errors/errors_test.go
@@ -0,0 +1,53 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errors_test
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+func TestNewEqual(t *testing.T) {
+	// Different allocations should not be equal.
+	if errors.New("abc") == errors.New("abc") {
+		t.Errorf(`New("abc") == New("abc")`)
+	}
+	if errors.New("abc") == errors.New("xyz") {
+		t.Errorf(`New("abc") == New("xyz")`)
+	}
+
+	// Same allocation should be equal to itself (not crash).
+	err := errors.New("jkl")
+	if err != err {
+		t.Errorf(`err != err`)
+	}
+}
+
+func TestErrorMethod(t *testing.T) {
+	err := errors.New("abc")
+	if err.Error() != "abc" {
+		t.Errorf(`New("abc").Error() = %q, want %q`, err.Error(), "abc")
+	}
+}
+
+func ExampleNew() {
+	err := errors.New("emit macho dwarf: elf header corrupted")
+	if err != nil {
+		fmt.Print(err)
+	}
+	// Output: emit macho dwarf: elf header corrupted
+}
+
+// The fmt package's Errorf function lets us use the package's formatting
+// features to create descriptive error messages.
+func ExampleNew_errorf() {
+	const name, id = "bimmler", 17
+	err := fmt.Errorf("user %q (id %d) not found", name, id)
+	if err != nil {
+		fmt.Print(err)
+	}
+	// Output: user "bimmler" (id 17) not found
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/errors/example_test.go b/internal/stdlib/testdata/v1.12.5/src/errors/example_test.go
new file mode 100644
index 0000000..5dc8841
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/errors/example_test.go
@@ -0,0 +1,34 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errors_test
+
+import (
+	"fmt"
+	"time"
+)
+
+// MyError is an error implementation that includes a time and message.
+type MyError struct {
+	When time.Time
+	What string
+}
+
+func (e MyError) Error() string {
+	return fmt.Sprintf("%v: %v", e.When, e.What)
+}
+
+func oops() error {
+	return MyError{
+		time.Date(1989, 3, 15, 22, 30, 0, 0, time.UTC),
+		"the file system has gone away",
+	}
+}
+
+func Example() {
+	if err := oops(); err != nil {
+		fmt.Println(err)
+	}
+	// Output: 1989-03-15 22:30:00 +0000 UTC: the file system has gone away
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/flag/example_test.go b/internal/stdlib/testdata/v1.12.5/src/flag/example_test.go
new file mode 100644
index 0000000..04a0d20
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/flag/example_test.go
@@ -0,0 +1,83 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// These examples demonstrate more intricate uses of the flag package.
+package flag_test
+
+import (
+	"errors"
+	"flag"
+	"fmt"
+	"strings"
+	"time"
+)
+
+// Example 1: A single string flag called "species" with default value "gopher".
+var species = flag.String("species", "gopher", "the species we are studying")
+
+// Example 2: Two flags sharing a variable, so we can have a shorthand.
+// The order of initialization is undefined, so make sure both use the
+// same default value. They must be set up with an init function.
+var gopherType string
+
+func init() {
+	const (
+		defaultGopher = "pocket"
+		usage         = "the variety of gopher"
+	)
+	flag.StringVar(&gopherType, "gopher_type", defaultGopher, usage)
+	flag.StringVar(&gopherType, "g", defaultGopher, usage+" (shorthand)")
+}
+
+// Example 3: A user-defined flag type, a slice of durations.
+type interval []time.Duration
+
+// String is the method to format the flag's value, part of the flag.Value interface.
+// The String method's output will be used in diagnostics.
+func (i *interval) String() string {
+	return fmt.Sprint(*i)
+}
+
+// Set is the method to set the flag value, part of the flag.Value interface.
+// Set's argument is a string to be parsed to set the flag.
+// It's a comma-separated list, so we split it.
+func (i *interval) Set(value string) error {
+	// If we wanted to allow the flag to be set multiple times,
+	// accumulating values, we would delete this if statement.
+	// That would permit usages such as
+	//	-deltaT 10s -deltaT 15s
+	// and other combinations.
+	if len(*i) > 0 {
+		return errors.New("interval flag already set")
+	}
+	for _, dt := range strings.Split(value, ",") {
+		duration, err := time.ParseDuration(dt)
+		if err != nil {
+			return err
+		}
+		*i = append(*i, duration)
+	}
+	return nil
+}
+
+// Define a flag to accumulate durations. Because it has a special type,
+// we need to use the Var function and therefore create the flag during
+// init.
+
+var intervalFlag interval
+
+func init() {
+	// Tie the command-line flag to the intervalFlag variable and
+	// set a usage message.
+	flag.Var(&intervalFlag, "deltaT", "comma-separated list of intervals to use between events")
+}
+
+func Example() {
+	// All the interesting pieces are with the variables declared above, but
+	// to enable the flag package to see the flags defined there, one must
+	// execute, typically at the start of main (not init!):
+	//	flag.Parse()
+	// We don't run it here because this is not a main function and
+	// the testing suite has already parsed the flags.
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/flag/example_value_test.go b/internal/stdlib/testdata/v1.12.5/src/flag/example_value_test.go
new file mode 100644
index 0000000..9d464c6
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/flag/example_value_test.go
@@ -0,0 +1,44 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package flag_test
+
+import (
+	"flag"
+	"fmt"
+	"net/url"
+)
+
+type URLValue struct {
+	URL *url.URL
+}
+
+func (v URLValue) String() string {
+	if v.URL != nil {
+		return v.URL.String()
+	}
+	return ""
+}
+
+func (v URLValue) Set(s string) error {
+	if u, err := url.Parse(s); err != nil {
+		return err
+	} else {
+		*v.URL = *u
+	}
+	return nil
+}
+
+var u = &url.URL{}
+
+func ExampleValue() {
+	fs := flag.NewFlagSet("ExampleValue", flag.ExitOnError)
+	fs.Var(&URLValue{u}, "url", "URL to parse")
+
+	fs.Parse([]string{"-url", "https://golang.org/pkg/flag/"})
+	fmt.Printf(`{scheme: %q, host: %q, path: %q}`, u.Scheme, u.Host, u.Path)
+
+	// Output:
+	// {scheme: "https", host: "golang.org", path: "/pkg/flag/"}
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/flag/export_test.go b/internal/stdlib/testdata/v1.12.5/src/flag/export_test.go
new file mode 100644
index 0000000..838cfaf
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/flag/export_test.go
@@ -0,0 +1,20 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package flag
+
+import "os"
+
+// Additional routines compiled into the package only during testing.
+
+var DefaultUsage = Usage
+
+// ResetForTesting clears all flag state and sets the usage function as directed.
+// After calling ResetForTesting, parse errors in flag handling will not
+// exit the program.
+func ResetForTesting(usage func()) {
+	CommandLine = NewFlagSet(os.Args[0], ContinueOnError)
+	CommandLine.Usage = commandLineUsage
+	Usage = usage
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/flag/flag.go b/internal/stdlib/testdata/v1.12.5/src/flag/flag.go
new file mode 100644
index 0000000..c312c62
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/flag/flag.go
@@ -0,0 +1,1041 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+	Package flag implements command-line flag parsing.
+
+	Usage
+
+	Define flags using flag.String(), Bool(), Int(), etc.
+
+	This declares an integer flag, -flagname, stored in the pointer ip, with type *int.
+		import "flag"
+		var ip = flag.Int("flagname", 1234, "help message for flagname")
+	If you like, you can bind the flag to a variable using the Var() functions.
+		var flagvar int
+		func init() {
+			flag.IntVar(&flagvar, "flagname", 1234, "help message for flagname")
+		}
+	Or you can create custom flags that satisfy the Value interface (with
+	pointer receivers) and couple them to flag parsing by
+		flag.Var(&flagVal, "name", "help message for flagname")
+	For such flags, the default value is just the initial value of the variable.
+
+	After all flags are defined, call
+		flag.Parse()
+	to parse the command line into the defined flags.
+
+	Flags may then be used directly. If you're using the flags themselves,
+	they are all pointers; if you bind to variables, they're values.
+		fmt.Println("ip has value ", *ip)
+		fmt.Println("flagvar has value ", flagvar)
+
+	After parsing, the arguments following the flags are available as the
+	slice flag.Args() or individually as flag.Arg(i).
+	The arguments are indexed from 0 through flag.NArg()-1.
+
+	Command line flag syntax
+
+	The following forms are permitted:
+
+		-flag
+		-flag=x
+		-flag x  // non-boolean flags only
+	One or two minus signs may be used; they are equivalent.
+	The last form is not permitted for boolean flags because the
+	meaning of the command
+		cmd -x *
+	where * is a Unix shell wildcard, will change if there is a file
+	called 0, false, etc. You must use the -flag=false form to turn
+	off a boolean flag.
+
+	Flag parsing stops just before the first non-flag argument
+	("-" is a non-flag argument) or after the terminator "--".
+
+	Integer flags accept 1234, 0664, 0x1234 and may be negative.
+	Boolean flags may be:
+		1, 0, t, f, T, F, true, false, TRUE, FALSE, True, False
+	Duration flags accept any input valid for time.ParseDuration.
+
+	The default set of command-line flags is controlled by
+	top-level functions.  The FlagSet type allows one to define
+	independent sets of flags, such as to implement subcommands
+	in a command-line interface. The methods of FlagSet are
+	analogous to the top-level functions for the command-line
+	flag set.
+*/
+package flag
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"reflect"
+	"sort"
+	"strconv"
+	"strings"
+	"time"
+)
+
+// ErrHelp is the error returned if the -help or -h flag is invoked
+// but no such flag is defined.
+var ErrHelp = errors.New("flag: help requested")
+
+// errParse is returned by Set if a flag's value fails to parse, such as with an invalid integer for Int.
+// It then gets wrapped through failf to provide more information.
+var errParse = errors.New("parse error")
+
+// errRange is returned by Set if a flag's value is out of range.
+// It then gets wrapped through failf to provide more information.
+var errRange = errors.New("value out of range")
+
+func numError(err error) error {
+	ne, ok := err.(*strconv.NumError)
+	if !ok {
+		return err
+	}
+	if ne.Err == strconv.ErrSyntax {
+		return errParse
+	}
+	if ne.Err == strconv.ErrRange {
+		return errRange
+	}
+	return err
+}
+
+// -- bool Value
+type boolValue bool
+
+func newBoolValue(val bool, p *bool) *boolValue {
+	*p = val
+	return (*boolValue)(p)
+}
+
+func (b *boolValue) Set(s string) error {
+	v, err := strconv.ParseBool(s)
+	if err != nil {
+		err = errParse
+	}
+	*b = boolValue(v)
+	return err
+}
+
+func (b *boolValue) Get() interface{} { return bool(*b) }
+
+func (b *boolValue) String() string { return strconv.FormatBool(bool(*b)) }
+
+func (b *boolValue) IsBoolFlag() bool { return true }
+
+// optional interface to indicate boolean flags that can be
+// supplied without "=value" text
+type boolFlag interface {
+	Value
+	IsBoolFlag() bool
+}
+
+// -- int Value
+type intValue int
+
+func newIntValue(val int, p *int) *intValue {
+	*p = val
+	return (*intValue)(p)
+}
+
+func (i *intValue) Set(s string) error {
+	v, err := strconv.ParseInt(s, 0, strconv.IntSize)
+	if err != nil {
+		err = numError(err)
+	}
+	*i = intValue(v)
+	return err
+}
+
+func (i *intValue) Get() interface{} { return int(*i) }
+
+func (i *intValue) String() string { return strconv.Itoa(int(*i)) }
+
+// -- int64 Value
+type int64Value int64
+
+func newInt64Value(val int64, p *int64) *int64Value {
+	*p = val
+	return (*int64Value)(p)
+}
+
+func (i *int64Value) Set(s string) error {
+	v, err := strconv.ParseInt(s, 0, 64)
+	if err != nil {
+		err = numError(err)
+	}
+	*i = int64Value(v)
+	return err
+}
+
+func (i *int64Value) Get() interface{} { return int64(*i) }
+
+func (i *int64Value) String() string { return strconv.FormatInt(int64(*i), 10) }
+
+// -- uint Value
+type uintValue uint
+
+func newUintValue(val uint, p *uint) *uintValue {
+	*p = val
+	return (*uintValue)(p)
+}
+
+func (i *uintValue) Set(s string) error {
+	v, err := strconv.ParseUint(s, 0, strconv.IntSize)
+	if err != nil {
+		err = numError(err)
+	}
+	*i = uintValue(v)
+	return err
+}
+
+func (i *uintValue) Get() interface{} { return uint(*i) }
+
+func (i *uintValue) String() string { return strconv.FormatUint(uint64(*i), 10) }
+
+// -- uint64 Value
+type uint64Value uint64
+
+func newUint64Value(val uint64, p *uint64) *uint64Value {
+	*p = val
+	return (*uint64Value)(p)
+}
+
+func (i *uint64Value) Set(s string) error {
+	v, err := strconv.ParseUint(s, 0, 64)
+	if err != nil {
+		err = numError(err)
+	}
+	*i = uint64Value(v)
+	return err
+}
+
+func (i *uint64Value) Get() interface{} { return uint64(*i) }
+
+func (i *uint64Value) String() string { return strconv.FormatUint(uint64(*i), 10) }
+
+// -- string Value
+type stringValue string
+
+func newStringValue(val string, p *string) *stringValue {
+	*p = val
+	return (*stringValue)(p)
+}
+
+func (s *stringValue) Set(val string) error {
+	*s = stringValue(val)
+	return nil
+}
+
+func (s *stringValue) Get() interface{} { return string(*s) }
+
+func (s *stringValue) String() string { return string(*s) }
+
+// -- float64 Value
+type float64Value float64
+
+func newFloat64Value(val float64, p *float64) *float64Value {
+	*p = val
+	return (*float64Value)(p)
+}
+
+func (f *float64Value) Set(s string) error {
+	v, err := strconv.ParseFloat(s, 64)
+	if err != nil {
+		err = numError(err)
+	}
+	*f = float64Value(v)
+	return err
+}
+
+func (f *float64Value) Get() interface{} { return float64(*f) }
+
+func (f *float64Value) String() string { return strconv.FormatFloat(float64(*f), 'g', -1, 64) }
+
+// -- time.Duration Value
+type durationValue time.Duration
+
+func newDurationValue(val time.Duration, p *time.Duration) *durationValue {
+	*p = val
+	return (*durationValue)(p)
+}
+
+func (d *durationValue) Set(s string) error {
+	v, err := time.ParseDuration(s)
+	if err != nil {
+		err = errParse
+	}
+	*d = durationValue(v)
+	return err
+}
+
+func (d *durationValue) Get() interface{} { return time.Duration(*d) }
+
+func (d *durationValue) String() string { return (*time.Duration)(d).String() }
+
+// Value is the interface to the dynamic value stored in a flag.
+// (The default value is represented as a string.)
+//
+// If a Value has an IsBoolFlag() bool method returning true,
+// the command-line parser makes -name equivalent to -name=true
+// rather than using the next command-line argument.
+//
+// Set is called once, in command line order, for each flag present.
+// The flag package may call the String method with a zero-valued receiver,
+// such as a nil pointer.
+type Value interface {
+	String() string
+	Set(string) error
+}
+
+// Getter is an interface that allows the contents of a Value to be retrieved.
+// It wraps the Value interface, rather than being part of it, because it
+// appeared after Go 1 and its compatibility rules. All Value types provided
+// by this package satisfy the Getter interface.
+type Getter interface {
+	Value
+	Get() interface{}
+}
+
+// ErrorHandling defines how FlagSet.Parse behaves if the parse fails.
+type ErrorHandling int
+
+// These constants cause FlagSet.Parse to behave as described if the parse fails.
+const (
+	ContinueOnError ErrorHandling = iota // Return a descriptive error.
+	ExitOnError                          // Call os.Exit(2).
+	PanicOnError                         // Call panic with a descriptive error.
+)
+
+// A FlagSet represents a set of defined flags. The zero value of a FlagSet
+// has no name and has ContinueOnError error handling.
+type FlagSet struct {
+	// Usage is the function called when an error occurs while parsing flags.
+	// The field is a function (not a method) that may be changed to point to
+	// a custom error handler. What happens after Usage is called depends
+	// on the ErrorHandling setting; for the command line, this defaults
+	// to ExitOnError, which exits the program after calling Usage.
+	Usage func()
+
+	name          string
+	parsed        bool
+	actual        map[string]*Flag
+	formal        map[string]*Flag
+	args          []string // arguments after flags
+	errorHandling ErrorHandling
+	output        io.Writer // nil means stderr; use out() accessor
+}
+
+// A Flag represents the state of a flag.
+type Flag struct {
+	Name     string // name as it appears on command line
+	Usage    string // help message
+	Value    Value  // value as set
+	DefValue string // default value (as text); for usage message
+}
+
+// sortFlags returns the flags as a slice in lexicographical sorted order.
+func sortFlags(flags map[string]*Flag) []*Flag {
+	list := make(sort.StringSlice, len(flags))
+	i := 0
+	for _, f := range flags {
+		list[i] = f.Name
+		i++
+	}
+	list.Sort()
+	result := make([]*Flag, len(list))
+	for i, name := range list {
+		result[i] = flags[name]
+	}
+	return result
+}
+
+// Output returns the destination for usage and error messages. os.Stderr is returned if
+// output was not set or was set to nil.
+func (f *FlagSet) Output() io.Writer {
+	if f.output == nil {
+		return os.Stderr
+	}
+	return f.output
+}
+
+// Name returns the name of the flag set.
+func (f *FlagSet) Name() string {
+	return f.name
+}
+
+// ErrorHandling returns the error handling behavior of the flag set.
+func (f *FlagSet) ErrorHandling() ErrorHandling {
+	return f.errorHandling
+}
+
+// SetOutput sets the destination for usage and error messages.
+// If output is nil, os.Stderr is used.
+func (f *FlagSet) SetOutput(output io.Writer) {
+	f.output = output
+}
+
+// VisitAll visits the flags in lexicographical order, calling fn for each.
+// It visits all flags, even those not set.
+func (f *FlagSet) VisitAll(fn func(*Flag)) {
+	for _, flag := range sortFlags(f.formal) {
+		fn(flag)
+	}
+}
+
+// VisitAll visits the command-line flags in lexicographical order, calling
+// fn for each. It visits all flags, even those not set.
+func VisitAll(fn func(*Flag)) {
+	CommandLine.VisitAll(fn)
+}
+
+// Visit visits the flags in lexicographical order, calling fn for each.
+// It visits only those flags that have been set.
+func (f *FlagSet) Visit(fn func(*Flag)) {
+	for _, flag := range sortFlags(f.actual) {
+		fn(flag)
+	}
+}
+
+// Visit visits the command-line flags in lexicographical order, calling fn
+// for each. It visits only those flags that have been set.
+func Visit(fn func(*Flag)) {
+	CommandLine.Visit(fn)
+}
+
+// Lookup returns the Flag structure of the named flag, returning nil if none exists.
+func (f *FlagSet) Lookup(name string) *Flag {
+	return f.formal[name]
+}
+
+// Lookup returns the Flag structure of the named command-line flag,
+// returning nil if none exists.
+func Lookup(name string) *Flag {
+	return CommandLine.formal[name]
+}
+
+// Set sets the value of the named flag.
+func (f *FlagSet) Set(name, value string) error {
+	flag, ok := f.formal[name]
+	if !ok {
+		return fmt.Errorf("no such flag -%v", name)
+	}
+	err := flag.Value.Set(value)
+	if err != nil {
+		return err
+	}
+	if f.actual == nil {
+		f.actual = make(map[string]*Flag)
+	}
+	f.actual[name] = flag
+	return nil
+}
+
+// Set sets the value of the named command-line flag.
+func Set(name, value string) error {
+	return CommandLine.Set(name, value)
+}
+
+// isZeroValue determines whether the string represents the zero
+// value for a flag.
+func isZeroValue(flag *Flag, value string) bool {
+	// Build a zero value of the flag's Value type, and see if the
+	// result of calling its String method equals the value passed in.
+	// This works unless the Value type is itself an interface type.
+	typ := reflect.TypeOf(flag.Value)
+	var z reflect.Value
+	if typ.Kind() == reflect.Ptr {
+		z = reflect.New(typ.Elem())
+	} else {
+		z = reflect.Zero(typ)
+	}
+	return value == z.Interface().(Value).String()
+}
+
+// UnquoteUsage extracts a back-quoted name from the usage
+// string for a flag and returns it and the un-quoted usage.
+// Given "a `name` to show" it returns ("name", "a name to show").
+// If there are no back quotes, the name is an educated guess of the
+// type of the flag's value, or the empty string if the flag is boolean.
+func UnquoteUsage(flag *Flag) (name string, usage string) {
+	// Look for a back-quoted name, but avoid the strings package.
+	usage = flag.Usage
+	for i := 0; i < len(usage); i++ {
+		if usage[i] == '`' {
+			for j := i + 1; j < len(usage); j++ {
+				if usage[j] == '`' {
+					name = usage[i+1 : j]
+					usage = usage[:i] + name + usage[j+1:]
+					return name, usage
+				}
+			}
+			break // Only one back quote; use type name.
+		}
+	}
+	// No explicit name, so use type if we can find one.
+	name = "value"
+	switch flag.Value.(type) {
+	case boolFlag:
+		name = ""
+	case *durationValue:
+		name = "duration"
+	case *float64Value:
+		name = "float"
+	case *intValue, *int64Value:
+		name = "int"
+	case *stringValue:
+		name = "string"
+	case *uintValue, *uint64Value:
+		name = "uint"
+	}
+	return
+}
+
+// PrintDefaults prints, to standard error unless configured otherwise, the
+// default values of all defined command-line flags in the set. See the
+// documentation for the global function PrintDefaults for more information.
+func (f *FlagSet) PrintDefaults() {
+	f.VisitAll(func(flag *Flag) {
+		s := fmt.Sprintf("  -%s", flag.Name) // Two spaces before -; see next two comments.
+		name, usage := UnquoteUsage(flag)
+		if len(name) > 0 {
+			s += " " + name
+		}
+		// Boolean flags of one ASCII letter are so common we
+		// treat them specially, putting their usage on the same line.
+		if len(s) <= 4 { // space, space, '-', 'x'.
+			s += "\t"
+		} else {
+			// Four spaces before the tab triggers good alignment
+			// for both 4- and 8-space tab stops.
+			s += "\n    \t"
+		}
+		s += strings.ReplaceAll(usage, "\n", "\n    \t")
+
+		if !isZeroValue(flag, flag.DefValue) {
+			if _, ok := flag.Value.(*stringValue); ok {
+				// put quotes on the value
+				s += fmt.Sprintf(" (default %q)", flag.DefValue)
+			} else {
+				s += fmt.Sprintf(" (default %v)", flag.DefValue)
+			}
+		}
+		fmt.Fprint(f.Output(), s, "\n")
+	})
+}
+
+// PrintDefaults prints, to standard error unless configured otherwise,
+// a usage message showing the default settings of all defined
+// command-line flags.
+// For an integer valued flag x, the default output has the form
+//	-x int
+//		usage-message-for-x (default 7)
+// The usage message will appear on a separate line for anything but
+// a bool flag with a one-byte name. For bool flags, the type is
+// omitted and if the flag name is one byte the usage message appears
+// on the same line. The parenthetical default is omitted if the
+// default is the zero value for the type. The listed type, here int,
+// can be changed by placing a back-quoted name in the flag's usage
+// string; the first such item in the message is taken to be a parameter
+// name to show in the message and the back quotes are stripped from
+// the message when displayed. For instance, given
+//	flag.String("I", "", "search `directory` for include files")
+// the output will be
+//	-I directory
+//		search directory for include files.
+//
+// To change the destination for flag messages, call CommandLine.SetOutput.
+func PrintDefaults() {
+	CommandLine.PrintDefaults()
+}
+
+// defaultUsage is the default function to print a usage message.
+func (f *FlagSet) defaultUsage() {
+	if f.name == "" {
+		fmt.Fprintf(f.Output(), "Usage:\n")
+	} else {
+		fmt.Fprintf(f.Output(), "Usage of %s:\n", f.name)
+	}
+	f.PrintDefaults()
+}
+
+// NOTE: Usage is not just defaultUsage(CommandLine)
+// because it serves (via godoc flag Usage) as the example
+// for how to write your own usage function.
+
+// Usage prints a usage message documenting all defined command-line flags
+// to CommandLine's output, which by default is os.Stderr.
+// It is called when an error occurs while parsing flags.
+// The function is a variable that may be changed to point to a custom function.
+// By default it prints a simple header and calls PrintDefaults; for details about the
+// format of the output and how to control it, see the documentation for PrintDefaults.
+// Custom usage functions may choose to exit the program; by default exiting
+// happens anyway as the command line's error handling strategy is set to
+// ExitOnError.
+var Usage = func() {
+	fmt.Fprintf(CommandLine.Output(), "Usage of %s:\n", os.Args[0])
+	PrintDefaults()
+}
+
+// NFlag returns the number of flags that have been set.
+func (f *FlagSet) NFlag() int { return len(f.actual) }
+
+// NFlag returns the number of command-line flags that have been set.
+func NFlag() int { return len(CommandLine.actual) }
+
+// Arg returns the i'th argument. Arg(0) is the first remaining argument
+// after flags have been processed. Arg returns an empty string if the
+// requested element does not exist.
+func (f *FlagSet) Arg(i int) string {
+	if i < 0 || i >= len(f.args) {
+		return ""
+	}
+	return f.args[i]
+}
+
+// Arg returns the i'th command-line argument. Arg(0) is the first remaining argument
+// after flags have been processed. Arg returns an empty string if the
+// requested element does not exist.
+func Arg(i int) string {
+	return CommandLine.Arg(i)
+}
+
+// NArg is the number of arguments remaining after flags have been processed.
+func (f *FlagSet) NArg() int { return len(f.args) }
+
+// NArg is the number of arguments remaining after flags have been processed.
+func NArg() int { return len(CommandLine.args) }
+
+// Args returns the non-flag arguments.
+func (f *FlagSet) Args() []string { return f.args }
+
+// Args returns the non-flag command-line arguments.
+func Args() []string { return CommandLine.args }
+
+// BoolVar defines a bool flag with specified name, default value, and usage string.
+// The argument p points to a bool variable in which to store the value of the flag.
+func (f *FlagSet) BoolVar(p *bool, name string, value bool, usage string) {
+	f.Var(newBoolValue(value, p), name, usage)
+}
+
+// BoolVar defines a bool flag with specified name, default value, and usage string.
+// The argument p points to a bool variable in which to store the value of the flag.
+func BoolVar(p *bool, name string, value bool, usage string) {
+	CommandLine.Var(newBoolValue(value, p), name, usage)
+}
+
+// Bool defines a bool flag with specified name, default value, and usage string.
+// The return value is the address of a bool variable that stores the value of the flag.
+func (f *FlagSet) Bool(name string, value bool, usage string) *bool {
+	p := new(bool)
+	f.BoolVar(p, name, value, usage)
+	return p
+}
+
+// Bool defines a bool flag with specified name, default value, and usage string.
+// The return value is the address of a bool variable that stores the value of the flag.
+func Bool(name string, value bool, usage string) *bool {
+	return CommandLine.Bool(name, value, usage)
+}
+
+// IntVar defines an int flag with specified name, default value, and usage string.
+// The argument p points to an int variable in which to store the value of the flag.
+func (f *FlagSet) IntVar(p *int, name string, value int, usage string) {
+	f.Var(newIntValue(value, p), name, usage)
+}
+
+// IntVar defines an int flag with specified name, default value, and usage string.
+// The argument p points to an int variable in which to store the value of the flag.
+func IntVar(p *int, name string, value int, usage string) {
+	CommandLine.Var(newIntValue(value, p), name, usage)
+}
+
+// Int defines an int flag with specified name, default value, and usage string.
+// The return value is the address of an int variable that stores the value of the flag.
+func (f *FlagSet) Int(name string, value int, usage string) *int {
+	p := new(int)
+	f.IntVar(p, name, value, usage)
+	return p
+}
+
+// Int defines an int flag with specified name, default value, and usage string.
+// The return value is the address of an int variable that stores the value of the flag.
+func Int(name string, value int, usage string) *int {
+	return CommandLine.Int(name, value, usage)
+}
+
+// Int64Var defines an int64 flag with specified name, default value, and usage string.
+// The argument p points to an int64 variable in which to store the value of the flag.
+func (f *FlagSet) Int64Var(p *int64, name string, value int64, usage string) {
+	f.Var(newInt64Value(value, p), name, usage)
+}
+
+// Int64Var defines an int64 flag with specified name, default value, and usage string.
+// The argument p points to an int64 variable in which to store the value of the flag.
+func Int64Var(p *int64, name string, value int64, usage string) {
+	CommandLine.Var(newInt64Value(value, p), name, usage)
+}
+
+// Int64 defines an int64 flag with specified name, default value, and usage string.
+// The return value is the address of an int64 variable that stores the value of the flag.
+func (f *FlagSet) Int64(name string, value int64, usage string) *int64 {
+	p := new(int64)
+	f.Int64Var(p, name, value, usage)
+	return p
+}
+
+// Int64 defines an int64 flag with specified name, default value, and usage string.
+// The return value is the address of an int64 variable that stores the value of the flag.
+func Int64(name string, value int64, usage string) *int64 {
+	return CommandLine.Int64(name, value, usage)
+}
+
+// UintVar defines a uint flag with specified name, default value, and usage string.
+// The argument p points to a uint variable in which to store the value of the flag.
+func (f *FlagSet) UintVar(p *uint, name string, value uint, usage string) {
+	f.Var(newUintValue(value, p), name, usage)
+}
+
+// UintVar defines a uint flag with specified name, default value, and usage string.
+// The argument p points to a uint variable in which to store the value of the flag.
+func UintVar(p *uint, name string, value uint, usage string) {
+	CommandLine.Var(newUintValue(value, p), name, usage)
+}
+
+// Uint defines a uint flag with specified name, default value, and usage string.
+// The return value is the address of a uint variable that stores the value of the flag.
+func (f *FlagSet) Uint(name string, value uint, usage string) *uint {
+	p := new(uint)
+	f.UintVar(p, name, value, usage)
+	return p
+}
+
+// Uint defines a uint flag with specified name, default value, and usage string.
+// The return value is the address of a uint variable that stores the value of the flag.
+func Uint(name string, value uint, usage string) *uint {
+	return CommandLine.Uint(name, value, usage)
+}
+
+// Uint64Var defines a uint64 flag with specified name, default value, and usage string.
+// The argument p points to a uint64 variable in which to store the value of the flag.
+func (f *FlagSet) Uint64Var(p *uint64, name string, value uint64, usage string) {
+	f.Var(newUint64Value(value, p), name, usage)
+}
+
+// Uint64Var defines a uint64 flag with specified name, default value, and usage string.
+// The argument p points to a uint64 variable in which to store the value of the flag.
+func Uint64Var(p *uint64, name string, value uint64, usage string) {
+	CommandLine.Var(newUint64Value(value, p), name, usage)
+}
+
+// Uint64 defines a uint64 flag with specified name, default value, and usage string.
+// The return value is the address of a uint64 variable that stores the value of the flag.
+func (f *FlagSet) Uint64(name string, value uint64, usage string) *uint64 {
+	p := new(uint64)
+	f.Uint64Var(p, name, value, usage)
+	return p
+}
+
+// Uint64 defines a uint64 flag with specified name, default value, and usage string.
+// The return value is the address of a uint64 variable that stores the value of the flag.
+func Uint64(name string, value uint64, usage string) *uint64 {
+	return CommandLine.Uint64(name, value, usage)
+}
+
+// StringVar defines a string flag with specified name, default value, and usage string.
+// The argument p points to a string variable in which to store the value of the flag.
+func (f *FlagSet) StringVar(p *string, name string, value string, usage string) {
+	f.Var(newStringValue(value, p), name, usage)
+}
+
+// StringVar defines a string flag with specified name, default value, and usage string.
+// The argument p points to a string variable in which to store the value of the flag.
+func StringVar(p *string, name string, value string, usage string) {
+	CommandLine.Var(newStringValue(value, p), name, usage)
+}
+
+// String defines a string flag with specified name, default value, and usage string.
+// The return value is the address of a string variable that stores the value of the flag.
+func (f *FlagSet) String(name string, value string, usage string) *string {
+	p := new(string)
+	f.StringVar(p, name, value, usage)
+	return p
+}
+
+// String defines a string flag with specified name, default value, and usage string.
+// The return value is the address of a string variable that stores the value of the flag.
+func String(name string, value string, usage string) *string {
+	return CommandLine.String(name, value, usage)
+}
+
+// Float64Var defines a float64 flag with specified name, default value, and usage string.
+// The argument p points to a float64 variable in which to store the value of the flag.
+func (f *FlagSet) Float64Var(p *float64, name string, value float64, usage string) {
+	f.Var(newFloat64Value(value, p), name, usage)
+}
+
+// Float64Var defines a float64 flag with specified name, default value, and usage string.
+// The argument p points to a float64 variable in which to store the value of the flag.
+func Float64Var(p *float64, name string, value float64, usage string) {
+	CommandLine.Var(newFloat64Value(value, p), name, usage)
+}
+
+// Float64 defines a float64 flag with specified name, default value, and usage string.
+// The return value is the address of a float64 variable that stores the value of the flag.
+func (f *FlagSet) Float64(name string, value float64, usage string) *float64 {
+	p := new(float64)
+	f.Float64Var(p, name, value, usage)
+	return p
+}
+
+// Float64 defines a float64 flag with specified name, default value, and usage string.
+// The return value is the address of a float64 variable that stores the value of the flag.
+func Float64(name string, value float64, usage string) *float64 {
+	return CommandLine.Float64(name, value, usage)
+}
+
+// DurationVar defines a time.Duration flag with specified name, default value, and usage string.
+// The argument p points to a time.Duration variable in which to store the value of the flag.
+// The flag accepts a value acceptable to time.ParseDuration.
+func (f *FlagSet) DurationVar(p *time.Duration, name string, value time.Duration, usage string) {
+	f.Var(newDurationValue(value, p), name, usage)
+}
+
+// DurationVar defines a time.Duration flag with specified name, default value, and usage string.
+// The argument p points to a time.Duration variable in which to store the value of the flag.
+// The flag accepts a value acceptable to time.ParseDuration.
+func DurationVar(p *time.Duration, name string, value time.Duration, usage string) {
+	CommandLine.Var(newDurationValue(value, p), name, usage)
+}
+
+// Duration defines a time.Duration flag with specified name, default value, and usage string.
+// The return value is the address of a time.Duration variable that stores the value of the flag.
+// The flag accepts a value acceptable to time.ParseDuration.
+func (f *FlagSet) Duration(name string, value time.Duration, usage string) *time.Duration {
+	p := new(time.Duration)
+	f.DurationVar(p, name, value, usage)
+	return p
+}
+
+// Duration defines a time.Duration flag with specified name, default value, and usage string.
+// The return value is the address of a time.Duration variable that stores the value of the flag.
+// The flag accepts a value acceptable to time.ParseDuration.
+func Duration(name string, value time.Duration, usage string) *time.Duration {
+	return CommandLine.Duration(name, value, usage)
+}
+
+// Var defines a flag with the specified name and usage string. The type and
+// value of the flag are represented by the first argument, of type Value, which
+// typically holds a user-defined implementation of Value. For instance, the
+// caller could create a flag that turns a comma-separated string into a slice
+// of strings by giving the slice the methods of Value; in particular, Set would
+// decompose the comma-separated string into the slice.
+func (f *FlagSet) Var(value Value, name string, usage string) {
+	// Remember the default value as a string; it won't change.
+	flag := &Flag{name, usage, value, value.String()}
+	_, alreadythere := f.formal[name]
+	if alreadythere {
+		var msg string
+		if f.name == "" {
+			msg = fmt.Sprintf("flag redefined: %s", name)
+		} else {
+			msg = fmt.Sprintf("%s flag redefined: %s", f.name, name)
+		}
+		fmt.Fprintln(f.Output(), msg)
+		panic(msg) // Happens only if flags are declared with identical names
+	}
+	if f.formal == nil {
+		f.formal = make(map[string]*Flag)
+	}
+	f.formal[name] = flag
+}
+
+// Var defines a flag with the specified name and usage string. The type and
+// value of the flag are represented by the first argument, of type Value, which
+// typically holds a user-defined implementation of Value. For instance, the
+// caller could create a flag that turns a comma-separated string into a slice
+// of strings by giving the slice the methods of Value; in particular, Set would
+// decompose the comma-separated string into the slice.
+func Var(value Value, name string, usage string) {
+	CommandLine.Var(value, name, usage)
+}
+
+// failf prints to standard error a formatted error and usage message and
+// returns the error.
+func (f *FlagSet) failf(format string, a ...interface{}) error {
+	err := fmt.Errorf(format, a...)
+	fmt.Fprintln(f.Output(), err)
+	f.usage()
+	return err
+}
+
+// usage calls the Usage method for the flag set if one is specified,
+// or the appropriate default usage function otherwise.
+func (f *FlagSet) usage() {
+	if f.Usage == nil {
+		f.defaultUsage()
+	} else {
+		f.Usage()
+	}
+}
+
+// parseOne parses one flag. It reports whether a flag was seen.
+func (f *FlagSet) parseOne() (bool, error) {
+	if len(f.args) == 0 {
+		return false, nil
+	}
+	s := f.args[0]
+	if len(s) < 2 || s[0] != '-' {
+		return false, nil
+	}
+	numMinuses := 1
+	if s[1] == '-' {
+		numMinuses++
+		if len(s) == 2 { // "--" terminates the flags
+			f.args = f.args[1:]
+			return false, nil
+		}
+	}
+	name := s[numMinuses:]
+	if len(name) == 0 || name[0] == '-' || name[0] == '=' {
+		return false, f.failf("bad flag syntax: %s", s)
+	}
+
+	// it's a flag. does it have an argument?
+	f.args = f.args[1:]
+	hasValue := false
+	value := ""
+	for i := 1; i < len(name); i++ { // equals cannot be first
+		if name[i] == '=' {
+			value = name[i+1:]
+			hasValue = true
+			name = name[0:i]
+			break
+		}
+	}
+	m := f.formal
+	flag, alreadythere := m[name] // BUG
+	if !alreadythere {
+		if name == "help" || name == "h" { // special case for nice help message.
+			f.usage()
+			return false, ErrHelp
+		}
+		return false, f.failf("flag provided but not defined: -%s", name)
+	}
+
+	if fv, ok := flag.Value.(boolFlag); ok && fv.IsBoolFlag() { // special case: doesn't need an arg
+		if hasValue {
+			if err := fv.Set(value); err != nil {
+				return false, f.failf("invalid boolean value %q for -%s: %v", value, name, err)
+			}
+		} else {
+			if err := fv.Set("true"); err != nil {
+				return false, f.failf("invalid boolean flag %s: %v", name, err)
+			}
+		}
+	} else {
+		// It must have a value, which might be the next argument.
+		if !hasValue && len(f.args) > 0 {
+			// value is the next arg
+			hasValue = true
+			value, f.args = f.args[0], f.args[1:]
+		}
+		if !hasValue {
+			return false, f.failf("flag needs an argument: -%s", name)
+		}
+		if err := flag.Value.Set(value); err != nil {
+			return false, f.failf("invalid value %q for flag -%s: %v", value, name, err)
+		}
+	}
+	if f.actual == nil {
+		f.actual = make(map[string]*Flag)
+	}
+	f.actual[name] = flag
+	return true, nil
+}
+
+// Parse parses flag definitions from the argument list, which should not
+// include the command name. Must be called after all flags in the FlagSet
+// are defined and before flags are accessed by the program.
+// The return value will be ErrHelp if -help or -h were set but not defined.
+func (f *FlagSet) Parse(arguments []string) error {
+	f.parsed = true
+	f.args = arguments
+	for {
+		seen, err := f.parseOne()
+		if seen {
+			continue
+		}
+		if err == nil {
+			break
+		}
+		switch f.errorHandling {
+		case ContinueOnError:
+			return err
+		case ExitOnError:
+			os.Exit(2)
+		case PanicOnError:
+			panic(err)
+		}
+	}
+	return nil
+}
+
+// Parsed reports whether f.Parse has been called.
+func (f *FlagSet) Parsed() bool {
+	return f.parsed
+}
+
+// Parse parses the command-line flags from os.Args[1:]. Must be called
+// after all flags are defined and before flags are accessed by the program.
+func Parse() {
+	// Ignore errors; CommandLine is set for ExitOnError.
+	CommandLine.Parse(os.Args[1:])
+}
+
+// Parsed reports whether the command-line flags have been parsed.
+func Parsed() bool {
+	return CommandLine.Parsed()
+}
+
+// CommandLine is the default set of command-line flags, parsed from os.Args.
+// The top-level functions such as BoolVar, Arg, and so on are wrappers for the
+// methods of CommandLine.
+var CommandLine = NewFlagSet(os.Args[0], ExitOnError)
+
+func init() {
+	// Override generic FlagSet default Usage with call to global Usage.
+	// Note: This is not CommandLine.Usage = Usage,
+	// because we want any eventual call to use any updated value of Usage,
+	// not the value it has when this line is run.
+	CommandLine.Usage = commandLineUsage
+}
+
+func commandLineUsage() {
+	Usage()
+}
+
+// NewFlagSet returns a new, empty flag set with the specified name and
+// error handling property. If the name is not empty, it will be printed
+// in the default usage message and in error messages.
+func NewFlagSet(name string, errorHandling ErrorHandling) *FlagSet {
+	f := &FlagSet{
+		name:          name,
+		errorHandling: errorHandling,
+	}
+	f.Usage = f.defaultUsage
+	return f
+}
+
+// Init sets the name and error handling property for a flag set.
+// By default, the zero FlagSet uses an empty name and the
+// ContinueOnError error handling policy.
+func (f *FlagSet) Init(name string, errorHandling ErrorHandling) {
+	f.name = name
+	f.errorHandling = errorHandling
+}
diff --git a/internal/stdlib/testdata/v1.12.5/src/flag/flag_test.go b/internal/stdlib/testdata/v1.12.5/src/flag/flag_test.go
new file mode 100644
index 0000000..dc1b2b5
--- /dev/null
+++ b/internal/stdlib/testdata/v1.12.5/src/flag/flag_test.go
@@ -0,0 +1,545 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package flag_test
+
+import (
+	"bytes"
+	. "flag"
+	"fmt"
+	"io"
+	"os"
+	"sort"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+)
+
+func boolString(s string) string {
+	if s == "0" {
+		return "false"
+	}
+	return "true"
+}
+
+func TestEverything(t *testing.T) {
+	ResetForTesting(nil)
+	Bool("test_bool", false, "bool value")
+	Int("test_int", 0, "int value")
+	Int64("test_int64", 0, "int64 value")
+	Uint("test_uint", 0, "uint value")
+	Uint64("test_uint64", 0, "uint64 value")
+	String("test_string", "0", "string value")
+	Float64("test_float64", 0, "float64 value")
+	Duration("test_duration", 0, "time.Duration value")
+
+	m := make(map[string]*Flag)
+	desired := "0"
+	visitor := func(f *Flag) {
+		if len(f.Name) > 5 && f.Name[0:5] == "test_" {
+			m[f.Name] = f
+			ok := false
+			switch {
+			case f.Value.String() == desired:
+				ok = true
+			case f.Name == "test_bool" && f.Value.String() == boolString(desired):
+				ok = true
+			case f.Name == "test_duration" && f.Value.String() == desired+"s":
+				ok = true
+			}
+			if !ok {
+				t.Error("Visit: bad value", f.Value.String(), "for", f.Name)
+			}
+		}
+	}
+	VisitAll(visitor)
+	if len(m) != 8 {
+		t.Error("VisitAll misses some flags")
+		for k, v := range m {
+			t.Log(k, *v)
+		}
+	}
+	m = make(map[string]*Flag)
+	Visit(visitor)
+	if len(m) != 0 {
+		t.Errorf("Visit sees unset flags")
+		for k, v := range m {
+			t.Log(k, *v)
+		}
+	}
+	// Now set all flags
+	Set("test_bool", "true")
+	Set("test_int", "1")
+	Set("test_int64", "1")
+	Set("test_uint", "1")
+	Set("test_uint64", "1")
+	Set("test_string", "1")
+	Set("test_float64", "1")
+	Set("test_duration", "1s")
+	desired = "1"
+	Visit(visitor)
+	if len(m) != 8 {
+		t.Error("Visit fails after set")
+		for k, v := range m {
+			t.Log(k, *v)
+		}
+	}
+	// Now test they're visited in sort order.
+	var flagNames []string
+	Visit(func(f *Flag) { flagNames = append(flagNames, f.Name) })
+	if !sort.StringsAreSorted(flagNames) {
+		t.Errorf("flag names not sorted: %v", flagNames)
+	}
+}
+
+func TestGet(t *testing.T) {
+	ResetForTesting(nil)
+	Bool("test_bool", true, "bool value")
+	Int("test_int", 1, "int value")
+	Int64("test_int64", 2, "int64 value")
+	Uint("test_uint", 3, "uint value")
+	Uint64("test_uint64", 4, "uint64 value")
+	String("test_string", "5", "string value")
+	Float64("test_float64", 6, "float64 value")
+	Duration("test_duration", 7, "time.Duration value")
+
+	visitor := func(f *Flag) {
+		if len(f.Name) > 5 && f.Name[0:5] == "test_" {
+			g, ok := f.Value.(Getter)
+			if !ok {
+				t.Errorf("Visit: value does not satisfy Getter: %T", f.Value)
+				return
+			}
+			switch f.Name {
+			case "test_bool":
+				ok = g.Get() == true
+			case "test_int":
+				ok = g.Get() == int(1)
+			case "test_int64":
+				ok = g.Get() == int64(2)
+			case "test_uint":
+				ok = g.Get() == uint(3)
+			case "test_uint64":
+				ok = g.Get() == uint64(4)
+			case "test_string":
+				ok = g.Get() == "5"
+			case "test_float64":
+				ok = g.Get() == float64(6)
+			case "test_duration":
+				ok = g.Get() == time.Duration(7)
+			}
+			if !ok {
+				t.Errorf("Visit: bad value %T(%v) for %s", g.Get(), g.Get(), f.Name)
+			}
+		}
+	}
+	VisitAll(visitor)
+}
+
+func TestUsage(t *testing.T) {
+	called := false
+	ResetForTesting(func() { called = true })
+	if CommandLine.Parse([]string{"-x"}) == nil {
+		t.Error("parse did not fail for unknown flag")
+	}
+	if !called {
+		t.Error("did not call Usage for unknown flag")
+	}
+}
+
+func testParse(f *FlagSet, t *testing.T) {
+	if f.Parsed() {
+		t.Error("f.Parse() = true before Parse")
+	}
+	boolFlag := f.Bool("bool", false, "bool value")
+	bool2Flag := f.Bool("bool2", false, "bool2 value")
+	intFlag := f.Int("int", 0, "int value")
+	int64Flag := f.Int64("int64", 0, "int64 value")
+	uintFlag := f.Uint("uint", 0, "uint value")
+	uint64Flag := f.Uint64("uint64", 0, "uint64 value")
+	stringFlag := f.String("string", "0", "string value")
+	float64Flag := f.Float64("float64", 0, "float64 value")
+	durationFlag := f.Duration("duration", 5*time.Second, "time.Duration value")
+	extra := "one-extra-argument"
+	args := []string{
+		"-bool",
+		"-bool2=true",
+		"--int", "22",
+		"--int64", "0x23",
+		"-uint", "24",
+		"--uint64", "25",
+		"-string", "hello",
+		"-float64", "2718e28",
+		"-duration", "2m",
+		extra,
+	}
+	if err := f.Parse(args); err != nil {
+		t.Fatal(err)
+	}
+	if !f.Parsed() {
+		t.Error("f.Parse() = false after Parse")
+	}
+	if *boolFlag != true {
+		t.Error("bool flag should be true, is ", *boolFlag)
+	}
+	if *bool2Flag != true {
+		t.Error("bool2 flag should be true, is ", *bool2Flag)
+	}
+	if *intFlag != 22 {
+		t.Error("int flag should be 22, is ", *intFlag)
+	}
+	if *int64Flag != 0x23 {
+		t.Error("int64 flag should be 0x23, is ", *int64Flag)
+	}
+	if *uintFlag != 24 {
+		t.Error("uint flag should be 24, is ", *uintFlag)
+	}
+	if *uint64Flag != 25 {
+		t.Error("uint64 flag should be 25, is ", *uint64Flag)
+	}
+	if *stringFlag != "hello" {
+		t.Error("string flag should be `hello`, is ", *stringFlag)
+	}
+	if *float64Flag != 2718e28 {
+		t.Error("float64 flag should be 2718e28, is ", *float64Flag)
+	}
+	if *durationFlag != 2*time.Minute {
+		t.Error("duration flag should be 2m, is ", *durationFlag)
+	}
+	if len(f.Args()) != 1 {
+		t.Error("expected one argument, got", len(f.Args()))
+	} else if f.Args()[0] != extra {
+		t.Errorf("expected argument %q got %q", extra, f.Args()[0])
+	}
+}
+
+func TestParse(t *testing.T) {
+	ResetForTesting(func() { t.Error("bad parse") })
+	testParse(CommandLine, t)
+}
+
+func TestFlagSetParse(t *testing.T) {
+	testParse(NewFlagSet("test", ContinueOnError), t)
+}
+
+// Declare a user-defined flag type.
+type flagVar []string
+
+func (f *flagVar) String() string {
+	return fmt.Sprint([]string(*f))
+}
+
+func (f *flagVar) Set(value string) error {
+	*f = append(*f, value)
+	return nil
+}
+
+func TestUserDefined(t *testing.T) {
+	var flags FlagSet
+	flags.Init("test", ContinueOnError)
+	var v flagVar
+	flags.Var(&v, "v", "usage")
+	if err := flags.Parse([]string{"-v", "1", "-v", "2", "-v=3"}); err != nil {
+		t.Error(err)
+	}
+	if len(v) != 3 {
+		t.Fatal("expected 3 args; got ", len(v))
+	}
+	expect := "[1 2 3]"
+	if v.String() != expect {
+		t.Errorf("expected value %q got %q", expect, v.String())
+	}
+}
+
+func TestUserDefinedForCommandLine(t *testing.T) {
+	const help = "HELP"
+	var result string
+	ResetForTesting(func() { result = help })
+	Usage()
+	if result != help {
+		t.Fatalf("got %q; expected %q", result, help)
+	}
+}
+
+// Declare a user-defined boolean flag type.
+type boolFlagVar struct {
+	count int
+}
+
+func (b *boolFlagVar) String() string {
+	return fmt.Sprintf("%d", b.count)
+}
+
+func (b *boolFlagVar) Set(value string) error {
+	if value == "true" {
+		b.count++
+	}
+	return nil
+}
+
+func (b *boolFlagVar) IsBoolFlag() bool {
+	return b.count < 4
+}
+
+func TestUserDefinedBool(t *testing.T) {
+	var flags FlagSet
+	flags.Init("test", ContinueOnError)
+	var b boolFlagVar
+	var err error
+	flags.Var(&b, "b", "usage")
+	if err = flags.Parse([]string{"-b", "-b", "-b", "-b=true", "-b=false", "-b", "barg", "-b"}); err != nil {
+		if b.count < 4 {
+			t.Error(err)
+		}
+	}
+
+	if b.count != 4 {
+		t.Errorf("want: %d; got: %d", 4, b.count)
+	}
+
+	if err == nil {
+		t.Error("expected error; got none")
+	}
+}
+
+func TestSetOutput(t *testing.T) {
+	var flags FlagSet
+	var buf bytes.Buffer
+	flags.SetOutput(&buf)
+	flags.Init("test", ContinueOnError)
+	flags.Parse([]string{"-unknown"})
+	if out := buf.String(); !strings.Contains(out, "-unknown") {
+		t.Logf("expected output mentioning unknown; got %q", out)
+	}
+}
+
+// This tests that one can reset the flags. This still works but not well, and is
+// superseded by FlagSet.
+func TestChangingArgs(t *testing.T) {
+	ResetForTesting(func() { t.Fatal("bad parse") })
+	oldArgs := os.Args
+	defer func() { os.Args = oldArgs }()
+	os.Args = []string{"cmd", "-before", "subcmd", "-after", "args"}
+	before := Bool("before", false, "")
+	if err := CommandLine.Parse(os.Args[1:]); err != nil {
+		t.Fatal(err)
+	}
+	cmd := Arg(0)
+	os.Args = Args()
+	after := Bool("after", false, "")
+	Parse()
+	args := Args()
+
+	if !*before || cmd != "subcmd" || !*after || len(args) != 1 || args[0] != "args" {
+		t.Fatalf("expected true subcmd true [args] got %v %v %v %v", *before, cmd, *after, args)
+	}
+}
+
+// Test that -help invokes the usage message and returns ErrHelp.
+func TestHelp(t *testing.T) {
+	var helpCalled = false
+	fs := NewFlagSet("help test", ContinueOnError)
+	fs.Usage = func() { helpCalled = true }
+	var flag bool
+	fs.BoolVar(&flag, "flag", false, "regular flag")
+	// Regular flag invocation should work
+	err := fs.Parse([]string{"-flag=true"})
+	if err != nil {
+		t.Fatal("expected no error; got ", err)
+	}
+	if !flag {
+		t.Error("flag was not set by -flag")
+	}
+	if helpCalled {
+		t.Error("help called for regular flag")
+		helpCalled = false // reset for next test
+	}
+	// Help flag should work as expected.
+	err = fs.Parse([]string{"-help"})
+	if err == nil {
+		t.Fatal("error expected")
+	}
+	if err != ErrHelp {
+		t.Fatal("expected ErrHelp; got ", err)
+	}
+	if !helpCalled {
+		t.Fatal("help was not called")
+	}
+	// If we define a help flag, that should override.
+	var help bool
+	fs.BoolVar(&help, "help", false, "help flag")
+	helpCalled = false
+	err = fs.Parse([]string{"-help"})
+	if err != nil {
+		t.Fatal("expected no error for defined -help; got ", err)
+	}
+	if helpCalled {
+		t.Fatal("help was called; should not have been for defined help flag")
+	}
+}
+
+const defaultOutput = `  -A	for bootstrapping, allow 'any' type
+  -Alongflagname
+    	disable bounds checking
+  -C	a boolean defaulting to true (default true)
+  -D path
+    	set relative path for local imports
+  -E string
+    	issue 23543 (default "0")
+  -F number
+    	a non-zero number (default 2.7)
+  -G float
+    	a float that defaults to zero
+  -M string
+    	a multiline
+    	help
+    	string
+  -N int
+    	a non-zero int (default 27)
+  -O	a flag
+    	multiline help string (default true)
+  -Z int
+    	an int that defaults to zero
+  -maxT timeout
+    	set timeout for dial
+`
+
+func TestPrintDefaults(t *testing.T) {
+	fs := NewFlagSet("print defaults test", ContinueOnError)
+	var buf bytes.Buffer
+	fs.SetOutput(&buf)
+	fs.Bool("A", false, "for bootstrapping, allow 'any' type")
+	fs.Bool("Alongflagname", false, "disable bounds checking")
+	fs.Bool("C", true, "a boolean defaulting to true")
+	fs.String("D", "", "set relative `path` for local imports")
+	fs.String("E", "0", "issue 23543")
+	fs.Float64("F", 2.7, "a non-zero `number`")
+	fs.Float64("G", 0, "a float that defaults to zero")
+	fs.String("M", "", "a multiline\nhelp\nstring")
+	fs.Int("N", 27, "a non-zero int")
+	fs.Bool("O", true, "a flag\nmultiline help string")
+	fs.Int("Z", 0, "an int that defaults to zero")
+	fs.Duration("maxT", 0, "set `timeout` for dial")
+	fs.PrintDefaults()
+	got := buf.String()
+	if got != defaultOutput {
+		t.Errorf("got %q want %q\n", got, defaultOutput)
+	}
+}
+
+// Issue 19230: validate range of Int and Uint flag values.
+func TestIntFlagOverflow(t *testing.T) {
+	if strconv.IntSize != 32 {
+		return
+	}
+	ResetForTesting(nil)
+	Int("i", 0, "")
+	Uint("u", 0, "")
+	if err := Set("i", "2147483648"); err == nil {
+		t.Error("unexpected success setting Int")
+	}
+	if err := Set("u", "4294967296"); err == nil {
+		t.Error("unexpected success setting Uint")
+	}
+}
+
+// Issue 20998: Usage should respect CommandLine.output.
+func TestUsageOutput(t *testing.T) {
+	ResetForTesting(DefaultUsage)
+	var buf bytes.Buffer
+	CommandLine.SetOutput(&buf)
+	defer func(old []string) { os.Args = old }(os.Args)
+	os.Args = []string{"app", "-i=1", "-unknown"}
+	Parse()
+	const want = "flag provided but not defined: -i\nUsage of app:\n"
+	if got := buf.String(); got != want {
+		t.Errorf("output = %q; want %q", got, want)
+	}
+}
+
+func TestGetters(t *testing.T) {
+	expectedName := "flag set"
+	expectedErrorHandling := ContinueOnError
+	expectedOutput := io.Writer(os.Stderr)
+	fs := NewFlagSet(expectedName, expectedErrorHandling)
+
+	if fs.Name() != expectedName {
+		t.Errorf("unexpected name: got %s, expected %s", fs.Name(), expectedName)
+	}
+	if fs.ErrorHandling() != expectedErrorHandling {
+		t.Errorf("unexpected ErrorHandling: got %d, expected %d", fs.ErrorHandling(), expectedErrorHandling)
+	}
+	if fs.Output() != expectedOutput {
+		t.Errorf("unexpected output: got %#v, expected %#v", fs.Output(), expectedOutput)
+	}
+
+	expectedName = "gopher"
+	expectedErrorHandling = ExitOnError
+	expectedOutput = os.Stdout
+	fs.Init(expectedName, expectedErrorHandling)
+	fs.SetOutput(expectedOutput)
+
+	if fs.Name() != expectedName {
+		t.Errorf("unexpected name: got %s, expected %s", fs.Name(), expectedName)
+	}
+	if fs.ErrorHandling() != expectedErrorHandling {
+		t.Errorf("unexpected ErrorHandling: got %d, expected %d", fs.ErrorHandling(), expectedErrorHandling)
+	}
+	if fs.Output() != expectedOutput {
+		t.Errorf("unexpected output: got %v, expected %v", fs.Output(), expectedOutput)
+	}
+}
+
+func TestParseError(t *testing.T) {
+	for _, typ := range []string{"bool", "int", "int64", "uint", "uint64", "float64", "duration"} {
+		fs := NewFlagSet("parse error test", ContinueOnError)
+		fs.SetOutput(io.Discard)
+		_ = fs.Bool("bool", false, "")
+		_ = fs.Int("int", 0, "")
+		_ = fs.Int64("int64", 0, "")
+		_ = fs.Uint("uint", 0, "")
+		_ = fs.Uint64("uint64", 0, "")
+		_ = fs.Float64("float64", 0, "")
+		_ = fs.Duration("duration", 0, "")
+		// Strings cannot give errors.
+		args := []string{"-" + typ + "=x"}
+		err := fs.Parse(args) // x is not a valid setting for any flag.
+		if err == nil {
+			t.Errorf("Parse(%q)=%v; expected parse error", args, err)
+			continue
+		}
+		if !strings.Contains(err.Error(), "invalid") || !strings.Contains(err.Error(), "parse error") {
+			t.Errorf("Parse(%q)=%v; expected parse error", args, err)
+		}
+	}
+}
+
+func TestRangeError(t *testing.T) {
+	bad := []string{
+		"-int=123456789012345678901",
+		"-int64=123456789012345678901",
+		"-uint=123456789012345678901",
+		"-uint64=123456789012345678901",
+		"-float64=1e1000",
+	}
+	for _, arg := range bad {
+		fs := NewFlagSet("parse error test", ContinueOnError)
+		fs.SetOutput(io.Discard)
+		_ = fs.Int("int", 0, "")
+		_ = fs.Int64("int64", 0, "")
+		_ = fs.Uint("uint", 0, "")
+		_ = fs.Uint64("uint64", 0, "")
+		_ = fs.Float64("float64", 0, "")
+		// Strings cannot give errors, and bools and durations do not return strconv.NumError.
+		err := fs.Parse([]string{arg})
+		if err == nil {
+			t.Errorf("Parse(%q)=%v; expected range error", arg, err)
+			continue
+		}
+		if !strings.Contains(err.Error(), "invalid") || !strings.Contains(err.Error(), "value out of range") {
+			t.Errorf("Parse(%q)=%v; expected range error", arg, err)
+		}
+	}
+}
diff --git a/internal/stdlib/testdata/v1.14.6/LICENSE b/internal/stdlib/testdata/v1.14.6/LICENSE
new file mode 100644
index 0000000..6a66aea
--- /dev/null
+++ b/internal/stdlib/testdata/v1.14.6/LICENSE
@@ -0,0 +1,27 @@
+Copyright (c) 2009 The Go Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+   * Redistributions of source code must retain the above copyright
+notice, this list of conditions and the following disclaimer.
+   * Redistributions in binary form must reproduce the above
+copyright notice, this list of conditions and the following disclaimer
+in the documentation and/or other materials provided with the
+distribution.
+   * Neither the name of Google Inc. nor the names of its
+contributors may be used to endorse or promote products derived from
+this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/internal/stdlib/testdata/v1.14.6/README.md b/internal/stdlib/testdata/v1.14.6/README.md
new file mode 100644
index 0000000..d6d2b9d
--- /dev/null
+++ b/internal/stdlib/testdata/v1.14.6/README.md
@@ -0,0 +1 @@
+# The Go Programming Language
diff --git a/internal/stdlib/testdata/v1.14.6/src/README.vendor b/internal/stdlib/testdata/v1.14.6/src/README.vendor
new file mode 100644
index 0000000..e74fc2f
--- /dev/null
+++ b/internal/stdlib/testdata/v1.14.6/src/README.vendor
@@ -0,0 +1,54 @@
+Vendoring in std and cmd
+========================
+
+The Go command maintains copies of external packages needed by the
+standard library in the src/vendor and src/cmd/vendor directories.
+
+In GOPATH mode, imports of vendored packages are resolved to these
+directories following normal vendor directory logic
+(see golang.org/s/go15vendor).
+
+In module mode, std and cmd are modules (defined in src/go.mod and
+src/cmd/go.mod). When a package outside std or cmd is imported
+by a package inside std or cmd, the import path is interpreted
+as if it had a "vendor/" prefix. For example, within "crypto/tls",
+an import of "golang.org/x/crypto/cryptobyte" resolves to
+"vendor/golang.org/x/crypto/cryptobyte". When a package with the
+same path is imported from a package outside std or cmd, it will
+be resolved normally. Consequently, a binary may be built with two
+copies of a package at different versions if the package is
+imported normally and vendored by the standard library.
+
+Vendored packages are internally renamed with a "vendor/" prefix
+to preserve the invariant that all packages have distinct paths.
+This is necessary to avoid compiler and linker conflicts. Adding
+a "vendor/" prefix also maintains the invariant that standard
+library packages begin with a dotless path element.
+
+The module requirements of std and cmd do not influence version
+selection in other modules. They are only considered when running
+module commands like 'go get' and 'go mod vendor' from a directory
+in GOROOT/src.
+
+Maintaining vendor directories
+==============================
+
+Before updating vendor directories, ensure that module mode is enabled.
+Make sure GO111MODULE=off is not set ('on' or 'auto' should work).
+
+Requirements may be added, updated, and removed with 'go get'.
+The vendor directory may be updated with 'go mod vendor'.
+A typical sequence might be:
+
+    cd src
+    go get -d golang.org/x/net@latest
+    go mod tidy
+    go mod vendor
+
+Use caution when passing '-u' to 'go get'. The '-u' flag updates
+modules providing all transitively imported packages, not only
+the module providing the target package.
+
+Note that 'go mod vendor' only copies packages that are transitively
+imported by packages in the current module. If a new package is needed,
+it should be imported before running 'go mod vendor'.
diff --git a/internal/stdlib/testdata/v1.14.6/src/cmd/README.vendor b/internal/stdlib/testdata/v1.14.6/src/cmd/README.vendor
new file mode 100644
index 0000000..ac0df5e
--- /dev/null
+++ b/internal/stdlib/testdata/v1.14.6/src/cmd/README.vendor
@@ -0,0 +1,2 @@
+See src/README.vendor for information on loading vendored packages
+and updating the vendor directory.
diff --git a/internal/stdlib/testdata/v1.14.6/src/context/context.go b/internal/stdlib/testdata/v1.14.6/src/context/context.go
new file mode 100644
index 0000000..b561968
--- /dev/null
+++ b/internal/stdlib/testdata/v1.14.6/src/context/context.go
@@ -0,0 +1,554 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package context defines the Context type, which carries deadlines,
+// cancellation signals, and other request-scoped values across API boundaries
+// and between processes.
+//
+// Incoming requests to a server should create a Context, and outgoing
+// calls to servers should accept a Context. The chain of function
+// calls between them must propagate the Context, optionally replacing
+// it with a derived Context created using WithCancel, WithDeadline,
+// WithTimeout, or WithValue. When a Context is canceled, all
+// Contexts derived from it are also canceled.
+//
+// The WithCancel, WithDeadline, and WithTimeout functions take a
+// Context (the parent) and return a derived Context (the child) and a
+// CancelFunc. Calling the CancelFunc cancels the child and its
+// children, removes the parent's reference to the child, and stops
+// any associated timers. Failing to call the CancelFunc leaks the
+// child and its children until the parent is canceled or the timer
+// fires. The go vet tool checks that CancelFuncs are used on all
+// control-flow paths.
+//
+// Programs that use Contexts should follow these rules to keep interfaces
+// consistent across packages and enable static analysis tools to check context
+// propagation:
+//
+// Do not store Contexts inside a struct type; instead, pass a Context
+// explicitly to each function that needs it. The Context should be the first
+// parameter, typically named ctx:
+//
+// 	func DoSomething(ctx context.Context, arg Arg) error {
+// 		// ... use ctx ...
+// 	}
+//
+// Do not pass a nil Context, even if a function permits it. Pass context.TODO
+// if you are unsure about which Context to use.
+//
+// Use context Values only for request-scoped data that transits processes and
+// APIs, not for passing optional parameters to functions.
+//
+// The same Context may be passed to functions running in different goroutines;
+// Contexts are safe for simultaneous use by multiple goroutines.
+//
+// See https://blog.golang.org/context for example code for a server that uses
+// Contexts.
+package context
+
+import (
+	"errors"
+	"internal/reflectlite"
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+// A Context carries a deadline, a cancellation signal, and other values across
+// API boundaries.
+//
+// Context's methods may be called by multiple goroutines simultaneously.
+type Context interface {
+	// Deadline returns the time when work done on behalf of this context
+	// should be canceled. Deadline returns ok==false when no deadline is
+	// set. Successive calls to Deadline return the same results.
+	Deadline() (deadline time.Time, ok bool)
+
+	// Done returns a channel that's closed when work done on behalf of this
+	// context should be canceled. Done may return nil if this context can
+	// never be canceled. Successive calls to Done return the same value.
+	// The close of the Done channel may happen asynchronously,
+	// after the cancel function returns.
+	//
+	// WithCancel arranges for Done to be closed when cancel is called;
+	// WithDeadline arranges for Done to be closed when the deadline
+	// expires; WithTimeout arranges for Done to be closed when the timeout
+	// elapses.
+	//
+	// Done is provided for use in select statements:
+	//
+	//  // Stream generates values with DoSomething and sends them to out
+	//  // until DoSomething returns an error or ctx.Done is closed.
+	//  func Stream(ctx context.Context, out chan<- Value) error {
+	//  	for {
+	//  		v, err := DoSomething(ctx)
+	//  		if err != nil {
+	//  			return err
+	//  		}
+	//  		select {
+	//  		case <-ctx.Done():
+	//  			return ctx.Err()
+	//  		case out <- v:
+	//  		}
+	//  	}
+	//  }
+	//
+	// See https://blog.golang.org/pipelines for more examples of how to use
+	// a Done channel for cancellation.
+	Done() <-chan struct{}
+
+	// If Done is not yet closed, Err returns nil.
+	// If Done is closed, Err returns a non-nil error explaining why:
+	// Canceled if the context was canceled
+	// or DeadlineExceeded if the context's deadline passed.
+	// After Err returns a non-nil error, successive calls to Err return the same error.
+	Err() error
+
+	// Value returns the value associated with this context for key, or nil
+	// if no value is associated with key. Successive calls to Value with
+	// the same key returns the same result.
+	//
+	// Use context values only for request-scoped data that transits
+	// processes and API boundaries, not for passing optional parameters to
+	// functions.
+	//
+	// A key identifies a specific value in a Context. Functions that wish
+	// to store values in Context typically allocate a key in a global
+	// variable then use that key as the argument to context.WithValue and
+	// Context.Value. A key can be any type that supports equality;
+	// packages should define keys as an unexported type to avoid
+	// collisions.
+	//
+	// Packages that define a Context key should provide type-safe accessors
+	// for the values stored using that key:
+	//
+	// 	// Package user defines a User type that's stored in Contexts.
+	// 	package user
+	//
+	// 	import "context"
+	//
+	// 	// User is the type of value stored in the Contexts.
+	// 	type User struct {...}
+	//
+	// 	// key is an unexported type for keys defined in this package.
+	// 	// This prevents collisions with keys defined in other packages.
+	// 	type key int
+	//
+	// 	// userKey is the key for user.User values in Contexts. It is
+	// 	// unexported; clients use user.NewContext and user.FromContext
+	// 	// instead of using this key directly.
+	// 	var userKey key
+	//
+	// 	// NewContext returns a new Context that carries value u.
+	// 	func NewContext(ctx context.Context, u *User) context.Context {
+	// 		return context.WithValue(ctx, userKey, u)
+	// 	}
+	//
+	// 	// FromContext returns the User value stored in ctx, if any.
+	// 	func FromContext(ctx context.Context) (*User, bool) {
+	// 		u, ok := ctx.Value(userKey).(*User)
+	// 		return u, ok
+	// 	}
+	Value(key interface{}) interface{}
+}
+
+// Canceled is the error returned by Context.Err when the context is canceled.
+var Canceled = errors.New("context canceled")
+
+// DeadlineExceeded is the error returned by Context.Err when the context's
+// deadline passes.
+var DeadlineExceeded error = deadlineExceededError{}
+
+type deadlineExceededError struct{}
+
+func (deadlineExceededError) Error() string   { return "context deadline exceeded" }
+func (deadlineExceededError) Timeout() bool   { return true }
+func (deadlineExceededError) Temporary() bool { return true }
+
+// An emptyCtx is never canceled, has no values, and has no deadline. It is not
+// struct{}, since vars of this type must have distinct addresses.
+type emptyCtx int
+
+func (*emptyCtx) Deadline() (deadline time.Time, ok bool) {
+	return
+}
+
+func (*emptyCtx) Done() <-chan struct{} {
+	return nil
+}
+
+func (*emptyCtx) Err() error {
+	return nil
+}
+
+func (*emptyCtx) Value(key interface{}) interface{} {
+	return nil
+}
+
+func (e *emptyCtx) String() string {
+	switch e {
+	case background:
+		return "context.Background"
+	case todo:
+		return "context.TODO"
+	}
+	return "unknown empty Context"
+}
+
+var (
+	background = new(emptyCtx)
+	todo       = new(emptyCtx)
+)
+
+// Background returns a non-nil, empty Context. It is never canceled, has no
+// values, and has no deadline. It is typically used by the main function,
+// initialization, and tests, and as the top-level Context for incoming
+// requests.
+func Background() Context {
+	return background
+}
+
+// TODO returns a non-nil, empty Context. Code should use context.TODO when
+// it's unclear which Context to use or it is not yet available (because the
+// surrounding function has not yet been extended to accept a Context
+// parameter).
+func TODO() Context {
+	return todo
+}
+
+// A CancelFunc tells an operation to abandon its work.
+// A CancelFunc does not wait for the work to stop.
+// A CancelFunc may be called by multiple goroutines simultaneously.
+// After the first call, subsequent calls to a CancelFunc do nothing.
+type CancelFunc func()
+
+// WithCancel returns a copy of parent with a new Done channel. The returned
+// context's Done channel is closed when the returned cancel function is called
+// or when the parent context's Done channel is closed, whichever happens first.
+//
+// Canceling this context releases resources associated with it, so code should
+// call cancel as soon as the operations running in this Context complete.
+func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
+	c := newCancelCtx(parent)
+	propagateCancel(parent, &c)
+	return &c, func() { c.cancel(true, Canceled) }
+}
+
+// newCancelCtx returns an initialized cancelCtx.
+func newCancelCtx(parent Context) cancelCtx {
+	return cancelCtx{Context: parent}
+}
+
+// goroutines counts the number of goroutines ever created; for testing.
+var goroutines int32
+
+// propagateCancel arranges for child to be canceled when parent is.
+func propagateCancel(parent Context, child canceler) {
+	done := parent.Done()
+	if done == nil {
+		return // parent is never canceled
+	}
+
+	select {
+	case <-done:
+		// parent is already canceled
+		child.cancel(false, parent.Err())
+		return
+	default:
+	}
+
+	if p, ok := parentCancelCtx(parent); ok {
+		p.mu.Lock()
+		if p.err != nil {
+			// parent has already been canceled
+			child.cancel(false, p.err)
+		} else {
+			if p.children == nil {
+				p.children = make(map[canceler]struct{})
+			}
+			p.children[child] = struct{}{}
+		}
+		p.mu.Unlock()
+	} else {
+		atomic.AddInt32(&goroutines, +1)
+		go func() {
+			select {
+			case <-parent.Done():
+				child.cancel(false, parent.Err())
+			case <-child.Done():
+			}
+		}()
+	}
+}
+
+// &cancelCtxKey is the key that a cancelCtx returns itself for.
+var cancelCtxKey int
+
+// parentCancelCtx returns the underlying *cancelCtx for parent.
+// It does this by looking up parent.Value(&cancelCtxKey) to find
+// the innermost enclosing *cancelCtx and then checking whether
+// parent.Done() matches that *cancelCtx. (If not, the *cancelCtx
+// has been wrapped in a custom implementation providing a
+// different done channel, in which case we should not bypass it.)
+func parentCancelCtx(parent Context) (*cancelCtx, bool) {
+	done := parent.Done()
+	if done == closedchan || done == nil {
+		return nil, false
+	}
+	p, ok := parent.Value(&cancelCtxKey).(*cancelCtx)
+	if !ok {
+		return nil, false
+	}
+	p.mu.Lock()
+	ok = p.done == done
+	p.mu.Unlock()
+	if !ok {
+		return nil, false
+	}
+	return p, true
+}
+
+// removeChild removes a context from its parent.
+func removeChild(parent Context, child canceler) {
+	p, ok := parentCancelCtx(parent)
+	if !ok {
+		return
+	}
+	p.mu.Lock()
+	if p.children != nil {
+		delete(p.children, child)
+	}
+	p.mu.Unlock()
+}
+
+// A canceler is a context type that can be canceled directly. The
+// implementations are *cancelCtx and *timerCtx.
+type canceler interface {
+	cancel(removeFromParent bool, err error)
+	Done() <-chan struct{}
+}
+
+// closedchan is a reusable closed channel.
+var closedchan = make(chan struct{})
+
+func init() {
+	close(closedchan)
+}
+
+// A cancelCtx can be canceled. When canceled, it also cancels any children
+// that implement canceler.
+type cancelCtx struct {
+	Context
+
+	mu       sync.Mutex            // protects following fields
+	done     chan struct{}         // created lazily, closed by first cancel call
+	children map[canceler]struct{} // set to nil by the first cancel call
+	err      error                 // set to non-nil by the first cancel call
+}
+
+func (c *cancelCtx) Value(key interface{}) interface{} {
+	if key == &cancelCtxKey {
+		return c
+	}
+	return c.Context.Value(key)
+}
+
+func (c *cancelCtx) Done() <-chan struct{} {
+	c.mu.Lock()
+	if c.done == nil {
+		c.done = make(chan struct{})
+	}
+	d := c.done
+	c.mu.Unlock()
+	return d
+}
+
+func (c *cancelCtx) Err() error {
+	c.mu.Lock()
+	err := c.err
+	c.mu.Unlock()
+	return err
+}
+
+type stringer interface {
+	String() string
+}
+
+func contextName(c Context) string {
+	if s, ok := c.(stringer); ok {
+		return s.String()
+	}
+	return reflectlite.TypeOf(c).String()
+}
+
+func (c *cancelCtx) String() string {
+	return contextName(c.Context) + ".WithCancel"
+}
+
+// cancel closes c.done, cancels each of c's children, and, if
+// removeFromParent is true, removes c from its parent's children.
+func (c *cancelCtx) cancel(removeFromParent bool, err error) {
+	if err == nil {
+		panic("context: internal error: missing cancel error")
+	}
+	c.mu.Lock()
+	if c.err != nil {
+		c.mu.Unlock()
+		return // already canceled
+	}
+	c.err = err
+	if c.done == nil {
+		c.done = closedchan
+	} else {
+		close(c.done)
+	}
+	for child := range c.children {
+		// NOTE: acquiring the child's lock while holding parent's lock.
+		child.cancel(false, err)
+	}
+	c.children = nil
+	c.mu.Unlock()
+
+	if removeFromParent {
+		removeChild(c.Context, c)
+	}
+}
+
+// WithDeadline returns a copy of the parent context with the deadline adjusted
+// to be no later than d. If the parent's deadline is already earlier than d,
+// WithDeadline(parent, d) is semantically equivalent to parent. The returned
+// context's Done channel is closed when the deadline expires, when the returned
+// cancel function is called, or when the parent context's Done channel is
+// closed, whichever happens first.
+//
+// Canceling this context releases resources associated with it, so code should
+// call cancel as soon as the operations running in this Context complete.
+func WithDeadline(parent Context, d time.Time) (Context, CancelFunc) {
+	if cur, ok := parent.Deadline(); ok && cur.Before(d) {
+		// The current deadline is already sooner than the new one.
+		return WithCancel(parent)
+	}
+	c := &timerCtx{
+		cancelCtx: newCancelCtx(parent),
+		deadline:  d,
+	}
+	propagateCancel(parent, c)
+	dur := time.Until(d)
+	if dur <= 0 {
+		c.cancel(true, DeadlineExceeded) // deadline has already passed
+		return c, func() { c.cancel(false, Canceled) }
+	}
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.err == nil {
+		c.timer = time.AfterFunc(dur, func() {
+			c.cancel(true, DeadlineExceeded)
+		})
+	}
+	return c, func() { c.cancel(true, Canceled) }
+}
+
+// A timerCtx carries a timer and a deadline. It embeds a cancelCtx to
+// implement Done and Err. It implements cancel by stopping its timer then
+// delegating to cancelCtx.cancel.
+type timerCtx struct {
+	cancelCtx
+	timer *time.Timer // Under cancelCtx.mu.
+
+	deadline time.Time
+}
+
+func (c *timerCtx) Deadline() (deadline time.Time, ok bool) {
+	return c.deadline, true
+}
+
+func (c *timerCtx) String() string {
+	return contextName(c.cancelCtx.Context) + ".WithDeadline(" +
+		c.deadline.String() + " [" +
+		time.Until(c.deadline).String() + "])"
+}
+
+func (c *timerCtx) cancel(removeFromParent bool, err error) {
+	c.cancelCtx.cancel(false, err)
+	if removeFromParent {
+		// Remove this timerCtx from its parent cancelCtx's children.
+		removeChild(c.cancelCtx.Context, c)
+	}
+	c.mu.Lock()
+	if c.timer != nil {
+		c.timer.Stop()
+		c.timer = nil
+	}
+	c.mu.Unlock()
+}
+
+// WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)).
+//
+// Canceling this context releases resources associated with it, so code should
+// call cancel as soon as the operations running in this Context complete:
+//
+// 	func slowOperationWithTimeout(ctx context.Context) (Result, error) {
+// 		ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)
+// 		defer cancel()  // releases resources if slowOperation completes before timeout elapses
+// 		return slowOperation(ctx)
+// 	}
+func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
+	return WithDeadline(parent, time.Now().Add(timeout))
+}
+
+// WithValue returns a copy of parent in which the value associated with key is
+// val.
+//
+// Use context Values only for request-scoped data that transits processes and
+// APIs, not for passing optional parameters to functions.
+//
+// The provided key must be comparable and should not be of type
+// string or any other built-in type to avoid collisions between
+// packages using context. Users of WithValue should define their own
+// types for keys. To avoid allocating when assigning to an
+// interface{}, context keys often have concrete type
+// struct{}. Alternatively, exported context key variables' static
+// type should be a pointer or interface.
+func WithValue(parent Context, key, val interface{}) Context {
+	if key == nil {
+		panic("nil key")
+	}
+	if !reflectlite.TypeOf(key).Comparable() {
+		panic("key is not comparable")
+	}
+	return &valueCtx{parent, key, val}
+}
+
+// A valueCtx carries a key-value pair. It implements Value for that key and
+// delegates all other calls to the embedded Context.
+type valueCtx struct {
+	Context
+	key, val interface{}
+}
+
+// stringify tries a bit to stringify v, without using fmt, since we don't
+// want context depending on the unicode tables. This is only used by
+// *valueCtx.String().
+func stringify(v interface{}) string {
+	switch s := v.(type) {
+	case stringer:
+		return s.String()
+	case string:
+		return s
+	}
+	return "<not Stringer>"
+}
+
+func (c *valueCtx) String() string {
+	return contextName(c.Context) + ".WithValue(type " +
+		reflectlite.TypeOf(c.key).String() +
+		", val " + stringify(c.val) + ")"
+}
+
+func (c *valueCtx) Value(key interface{}) interface{} {
+	if c.key == key {
+		return c.val
+	}
+	return c.Context.Value(key)
+}
diff --git a/internal/stdlib/testdata/v1.14.6/src/errors/errors.go b/internal/stdlib/testdata/v1.14.6/src/errors/errors.go
new file mode 100644
index 0000000..b8a4692
--- /dev/null
+++ b/internal/stdlib/testdata/v1.14.6/src/errors/errors.go
@@ -0,0 +1,20 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package errors implements functions to manipulate errors.
+package errors
+
+// New returns an error that formats as the given text.
+func New(text string) error {
+	return &errorString{text}
+}
+
+// errorString is a trivial implementation of error.
+type errorString struct {
+	s string
+}
+
+func (e *errorString) Error() string {
+	return e.s
+}
diff --git a/internal/stdlib/testdata/v1.14.6/src/errors/errors_test.go b/internal/stdlib/testdata/v1.14.6/src/errors/errors_test.go
new file mode 100644
index 0000000..cf4df90
--- /dev/null
+++ b/internal/stdlib/testdata/v1.14.6/src/errors/errors_test.go
@@ -0,0 +1,53 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errors_test
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+func TestNewEqual(t *testing.T) {
+	// Different allocations should not be equal.
+	if errors.New("abc") == errors.New("abc") {
+		t.Errorf(`New("abc") == New("abc")`)
+	}
+	if errors.New("abc") == errors.New("xyz") {
+		t.Errorf(`New("abc") == New("xyz")`)
+	}
+
+	// Same allocation should be equal to itself (not crash).
+	err := errors.New("jkl")
+	if err != err {
+		t.Errorf(`err != err`)
+	}
+}
+
+func TestErrorMethod(t *testing.T) {
+	err := errors.New("abc")
+	if err.Error() != "abc" {
+		t.Errorf(`New("abc").Error() = %q, want %q`, err.Error(), "abc")
+	}
+}
+
+func ExampleNew() {
+	err := errors.New("emit macho dwarf: elf header corrupted")
+	if err != nil {
+		fmt.Print(err)
+	}
+	// Output: emit macho dwarf: elf header corrupted
+}
+
+// The fmt package's Errorf function lets us use the package's formatting
+// features to create descriptive error messages.
+func ExampleNew_errorf() {
+	const name, id = "bimmler", 17
+	err := fmt.Errorf("user %q (id %d) not found", name, id)
+	if err != nil {
+		fmt.Print(err)
+	}
+	// Output: user "bimmler" (id 17) not found
+}
diff --git a/internal/stdlib/testdata/v1.3.2/AUTHORS b/internal/stdlib/testdata/v1.3.2/AUTHORS
new file mode 100644
index 0000000..d4fbbd1
--- /dev/null
+++ b/internal/stdlib/testdata/v1.3.2/AUTHORS
@@ -0,0 +1,425 @@
+# This is the official list of Go authors for copyright purposes.
+# This file is distinct from the CONTRIBUTORS files.
+# See the latter for an explanation.
+
+# Names should be added to this file as
+#	Name or Organization <email address>
+# The email address is not required for organizations.
+
+# Please keep the list sorted.
+
+Aaron France <aaron.l.france@gmail.com>
+Abhinav Gupta <abhinav.g90@gmail.com>
+Adrian Nos <nos.adrian@gmail.com>
+Adrian O'Grady <elpollouk@gmail.com>
+Adrien Bustany <adrien-xx-google@bustany.org>
+Akshat Kumar <seed@mail.nanosouffle.net>
+Albert Strasheim <fullung@gmail.com>
+Alberto García Hierro <alberto@garciahierro.com> <alberto.garcia.hierro@gmail.com>
+Aleksandar Dezelin <dezelin@gmail.com>
+Alex A Skinner <alex@lx.lc>
+Alex Brainman <alex.brainman@gmail.com>
+Alex Jin <toalexjin@gmail.com>
+Alexander Larsson <alexander.larsson@gmail.com>
+Alexander Orlov <alexander.orlov@loxal.net>
+Alexander Reece <awreece@gmail.com>
+Alexander Surma <surma@surmair.de>
+Alexander Zhavnerchik <alex.vizor@gmail.com>
+Alexandre Normand <alexandre.normand@gmail.com>
+Alexei Sholik <alcosholik@gmail.com>
+Alexey Borzenkov <snaury@gmail.com>
+Alexey Palazhchenko <alexey.palazhchenko@gmail.com>
+Amir Mohammad Saied <amir@gluegadget.com>
+Amrut Joshi <amrut.joshi@gmail.com>
+Andrei Vieru <euvieru@gmail.com>
+Andrew Balholm <andybalholm@gmail.com>
+Andrew Bonventre <andybons@chromium.org>
+Andrew Harding <andrew@spacemonkey.com>
+Andrew Lutomirski <andy@luto.us>
+Andrew Pritchard <awpritchard@gmail.com>
+Andrew Radev <andrey.radev@gmail.com>
+Andrew Skiba <skibaa@gmail.com>
+Andrew Szeto <andrew@jabagawee.com>
+Andrew Wilkins <axwalk@gmail.com>
+Andrey Mirtchovski <mirtchovski@gmail.com>
+Andriy Lytvynov <lytvynov.a.v@gmail.com>
+Andy Davis <andy@bigandian.com>
+Anfernee Yongkun Gui <anfernee.gui@gmail.com>
+Anh Hai Trinh <anh.hai.trinh@gmail.com>
+Anschel Schaffer-Cohen <anschelsc@gmail.com>
+Anthony Eufemio <anthony.eufemio@gmail.com>
+Anthony Martin <ality@pbrane.org>
+Anthony Starks <ajstarks@gmail.com>
+Apisak Darakananda <pongad@gmail.com>
+Aram Hăvărneanu <aram@mgk.ro>
+Arnaud Ysmal <arnaud.ysmal@gmail.com>
+Arne Hormann <arnehormann@gmail.com>
+Aron Nopanen <aron.nopanen@gmail.com>
+Arvindh Rajesh Tamilmani <art@a-30.net>
+Ato Araki <ato.araki@gmail.com>
+Aulus Egnatius Varialus <varialus@gmail.com>
+Ben Olive <sionide21@gmail.com>
+Benjamin Black <b@b3k.us>
+Benny Siegert <bsiegert@gmail.com>
+Berengar Lehr <berengar.lehr@gmx.de>
+Billie Harold Cleek <bhcleek@gmail.com>
+Bjorn Tillenius <bjorn@tillenius.me>
+Bjorn Tipling <bjorn.tipling@gmail.com>
+Blake Mizerany <blake.mizerany@gmail.com>
+Bobby Powers <bobbypowers@gmail.com>
+Brendan Daniel Tracey <tracey.brendan@gmail.com>
+Brian Dellisanti <briandellisanti@gmail.com>
+Brian G. Merrell <bgmerrell@gmail.com>
+Brian Gitonga Marete <marete@toshnix.com>
+Brian Ketelsen <bketelsen@gmail.com>
+Caine Tighe <arctanofyourface@gmail.com>
+Caleb Spare <cespare@gmail.com>
+Carl Chatfield <carlchatfield@gmail.com>
+Carlos Castillo <cookieo9@gmail.com>
+Case Nelson <case.nelson@gmail.com>
+Casey Marshall <casey.marshall@gmail.com>
+Cezar Sá Espinola <cezarsa@gmail.com>
+ChaiShushan <chaishushan@gmail.com>
+Charles L. Dorian <cldorian@gmail.com>
+Charles Lee <zombie.fml@gmail.com>
+Chris Dollin <ehog.hedge@gmail.com>
+Chris Farmiloe <chrisfarms@gmail.com>
+Chris Howey <howeyc@gmail.com>
+Chris Jones <chris@cjones.org>
+Chris Lennert <calennert@gmail.com>
+Christian Himpel <chressie@googlemail.com>
+Christine Hansmann <chhansmann@gmail.com>
+Christoffer Buchholz <christoffer.buchholz@gmail.com>
+Christoph Hack <christoph@tux21b.org>
+Christopher Cahoon <chris.cahoon@gmail.com>
+Christopher Nielsen <m4dh4tt3r@gmail.com>
+Christopher Redden <christopher.redden@gmail.com>
+Christopher Wedgwood <cw@f00f.org>
+Clement Skau <clementskau@gmail.com>
+Conrad Meyer <cemeyer@cs.washington.edu>
+Corey Thomasson <cthom.lists@gmail.com>
+Damian Gryski <dgryski@gmail.com>
+Dan Callahan <dan.callahan@gmail.com>
+Dan Peterson <dpiddy@gmail.com>
+Dan Sinclair <dan.sinclair@gmail.com>
+Daniel Fleischman <danielfleischman@gmail.com>
+Daniel Krech <eikeon@eikeon.com>
+Daniel Lidén <daniel.liden.87@gmail.com>
+Daniel Morsing <daniel.morsing@gmail.com>
+Daniel Theophanes <kardianos@gmail.com>
+Darren Elwood <darren@textnode.com>
+Dave Cheney <dave@cheney.net>
+David Bürgin <676c7473@gmail.com>
+David Calavera <david.calavera@gmail.com>
+David du Colombier <0intro@gmail.com>
+David Forsythe <dforsythe@gmail.com>
+David G. Andersen <dave.andersen@gmail.com>
+David Jakob Fritz <david.jakob.fritz@gmail.com>
+David Thomas <davidthomas426@gmail.com>
+David Titarenco <david.titarenco@gmail.com>
+Dean Prichard <dean.prichard@gmail.com>
+Denis Brandolini <denis.brandolini@gmail.com>
+Devon H. O'Dell <devon.odell@gmail.com>
+Dhiru Kholia <dhiru.kholia@gmail.com>
+Dimitri Tcaciuc <dtcaciuc@gmail.com>
+Dmitri Shuralyov <shurcooL@gmail.com>
+Dmitriy Shelenin <deemok@googlemail.com> <deemok@gmail.com>
+Dmitry Chestnykh <dchest@gmail.com>
+Dominik Honnef <dominik.honnef@gmail.com>
+Donovan Hide <donovanhide@gmail.com>
+Dropbox, Inc.
+Duncan Holm <mail@frou.org>
+Dustin Sallings <dsallings@gmail.com>
+Dustin Shields-Cloues <dcloues@gmail.com>
+Eden Li <eden.li@gmail.com>
+Egon Elbre <egonelbre@gmail.com>
+Ehren Kret <ehren.kret@gmail.com>
+Eivind Uggedal <eivind@uggedal.com>
+Elias Naur <elias.naur@gmail.com>
+Emil Hessman <c.emil.hessman@gmail.com>
+Eoghan Sherry <ejsherry@gmail.com>
+Eric Clark <zerohp@gmail.com>
+Eric Milliken <emilliken@gmail.com>
+Eric Roshan-Eisner <eric.d.eisner@gmail.com>
+Erik St. Martin <alakriti@gmail.com>
+Erik Westrup <erik.westrup@gmail.com>
+Esko Luontola <esko.luontola@gmail.com>
+Evan Shaw <chickencha@gmail.com>
+Ewan Chou <coocood@gmail.com>
+Fabrizio Milo <mistobaan@gmail.com>
+Fan Hongjian <fan.howard@gmail.com>
+Fazlul Shahriar <fshahriar@gmail.com>
+Felix Geisendörfer <haimuiba@gmail.com>
+Firmansyah Adiputra <frm.adiputra@gmail.com>
+Florian Uekermann <florian@uekermann-online.de>
+Florian Weimer <fw@deneb.enyo.de>
+Francisco Souza <franciscossouza@gmail.com>
+Frederick Kelly Mayle III <frederickmayle@gmail.com>
+Fredrik Enestad <fredrik.enestad@soundtrackyourbrand.com>
+Frithjof Schulze <schulze@math.uni-hannover.de> <sfrithjof@gmail.com>
+Gary Burd <gary@beagledreams.com>
+Gautham Thambidorai <gautham.dorai@gmail.com>
+Georg Reinke <guelfey@gmail.com>
+Gerasimos Dimitriadis <gedimitr@gmail.com>
+Gideon Jan-Wessel Redelinghuys <gjredelinghuys@gmail.com>
+Giles Lean <giles.lean@pobox.com>
+Google Inc.
+Gordon Klaus <gordon.klaus@gmail.com>
+Graham King <graham4king@gmail.com>
+Graham Miller <graham.miller@gmail.com>
+Greg Ward <greg@gerg.ca>
+Guillaume J. Charmes <guillaume@charmes.net>
+Gustav Paul <gustav.paul@gmail.com>
+Gustavo Niemeyer <gustavo@niemeyer.net>
+Gwenael Treguier <gwenn.kahz@gmail.com>
+Harley Laue <losinggeneration@gmail.com>
+Hector Chu <hectorchu@gmail.com>
+Henrik Edwards <henrik.edwards@gmail.com>
+Herbert Georg Fischer <herbert.fischer@gmail.com>
+Hong Ruiqi <hongruiqi@gmail.com>
+Icarus Sparry <golang@icarus.freeuk.com>
+Ingo Oeser <nightlyone@googlemail.com>
+Isaac Wagner <ibw@isaacwagner.me>
+Jakob Borg <jakob@nym.se>
+Jakub Ryszard Czarnowicz <j.czarnowicz@gmail.com>
+James David Chalfant <james.chalfant@gmail.com>
+James Fysh <james.fysh@gmail.com>
+James Gray <james@james4k.com>
+James Meneghello <rawrz0r@gmail.com>
+James P. Cooper <jamespcooper@gmail.com>
+James Toy <nil@opensesame.st>
+James Whitehead <jnwhiteh@gmail.com>
+Jan H. Hosang <jan.hosang@gmail.com>
+Jan Mercl <0xjnml@gmail.com>
+Jan Mercl <befelemepeseveze@gmail.com>
+Jan Newmarch <jan.newmarch@gmail.com>
+Jan Ziak <0xe2.0x9a.0x9b@gmail.com>
+Jani Monoses <jani.monoses@ubuntu.com>
+Jaroslavas Počepko <jp@webmaster.ms>
+Jason Del Ponte <delpontej@gmail.com>
+Jason Travis <infomaniac7@gmail.com>
+Jay Weisskopf <jay@jayschwa.net>
+Jeff Hodges <jeff@somethingsimilar.com>
+Jeff R. Allen <jra@nella.org>
+Jeff Sickel <jas@corpus-callosum.com>
+Jeff Wendling <jeff@spacemonkey.com>
+Jeremy Jackins <jeremyjackins@gmail.com>
+Jim McGrath <jimmc2@gmail.com>
+Jimmy Zelinskie <jimmyzelinskie@gmail.com>
+Jingcheng Zhang <diogin@gmail.com>
+Joakim Sernbrant <serbaut@gmail.com>
+Joe Poirier <jdpoirier@gmail.com>
+John Asmuth <jasmuth@gmail.com>
+John C Barstow <jbowtie@amathaine.com>
+John Graham-Cumming <jgc@jgc.org> <jgrahamc@gmail.com>
+John Howard Palevich <jack.palevich@gmail.com>
+John Shahid <jvshahid@gmail.com>
+Jonathan Gold <jgold.bg@gmail.com>
+Jonathan Mark <jhmark@xenops.com>
+Jonathan Rudenberg <jonathan@titanous.com>
+Jonathan Wills <runningwild@gmail.com>
+Jongmin Kim <atomaths@gmail.com>
+Jose Luis Vázquez González <josvazg@gmail.com>
+Joseph Holsten <joseph@josephholsten.com>
+Josh Bleecher Snyder <josharian@gmail.com>
+Josh Goebel <dreamer3@gmail.com>
+Josh Holland <jrh@joshh.co.uk>
+Joshua Chase <jcjoshuachase@gmail.com>
+Jukka-Pekka Kekkonen <karatepekka@gmail.com>
+Julian Phillips <julian@quantumfyre.co.uk>
+Julien Schmidt <google@julienschmidt.com>
+Kai Backman <kaib@golang.org>
+Kamil Kisiel <kamil@kamilkisiel.net> <kamil.kisiel@gmail.com>
+Katrina Owen <katrina.owen@gmail.com>
+Kei Son <hey.calmdown@gmail.com>
+Keith Rarick <kr@xph.us>
+Kelsey Hightower <kelsey.hightower@gmail.com>
+Kelvin Foo Chuan Lyi <vmirage@gmail.com>
+Ken Friedenbach <kenliz@cruzio.com>
+Ken Rockot <ken@oz.gs>
+Kevin Ballard <kevin@sb.org>
+Kyle Consalus <consalus@gmail.com>
+Kyle Isom <kyle@gokyle.net>
+Kyle Lemons <kyle@kylelemons.net>
+L Campbell <unpantsu@gmail.com>
+Lai Jiangshan <eag0628@gmail.com>
+Linaro Limited
+Lorenzo Stoakes <lstoakes@gmail.com>
+Luca Greco <luca.greco@alcacoop.it>
+Lucio De Re <lucio.dere@gmail.com>
+Luit van Drongelen <luitvd@gmail.com>
+Luka Zakrajšek <tr00.g33k@gmail.com>
+Luke Curley <qpingu@gmail.com>
+Marc Weistroff <marc@weistroff.net>
+Marco Hennings <marco.hennings@freiheit.com>
+Marko Juhani Silokunnas <marko.silokunnas@gmail.com>
+Marko Tiikkaja <marko@joh.to>
+Markus Duft <markus.duft@salomon.at>
+Markus Sonderegger <marraison@gmail.com>
+Markus Zimmermann <zimmski@gmail.com>
+Martin Neubauer <m.ne@gmx.net>
+Martin Olsson <martin@minimum.se>
+Mateusz Czapliński <czapkofan@gmail.com>
+Mathieu Lonjaret <mathieu.lonjaret@gmail.com>
+Mats Lidell <mats.lidell@cag.se>
+Matt Aimonetti <mattaimonetti@gmail.com>
+Matt Jibson <matt.jibson@gmail.com>
+Matt Joiner <anacrolix@gmail.com>
+Matt Reiferson <mreiferson@gmail.com>
+Matthew Cottingham <mattcottingham@gmail.com>
+Matthew Horsnell <matthew.horsnell@gmail.com>
+Maxim Khitrov <max@mxcrypt.com>
+Micah Stetson <micah.stetson@gmail.com>
+Michael Chaten <mchaten@gmail.com>
+Michael Elkins <michael.elkins@gmail.com>
+Michael Fraenkel <michael.fraenkel@gmail.com>
+Michael Gehring <mg@ebfe.org> <gnirheg.leahcim@gmail.com>
+Michael Hoisie <hoisie@gmail.com>
+Michael Lewis <mikelikespie@gmail.com>
+Michael Pearson <mipearson@gmail.com>
+Michael Stapelberg <michael@stapelberg.de>
+Michael Teichgräber <mteichgraeber@gmx.de>
+Michał Derkacz <ziutek@lnet.pl>
+Miek Gieben <miek@miek.nl>
+Mihai Borobocea <MihaiBorobocea@gmail.com>
+Mikael Tillenius <mikti42@gmail.com>
+Mike Andrews <mra@xoba.com>
+Mike Rosset <mike.rosset@gmail.com>
+Mikhail Panchenko <m@mihasya.com>
+Miki Tebeka <miki.tebeka@gmail.com>
+Mikio Hara <mikioh.mikioh@gmail.com>
+Mikkel Krautz <mikkel@krautz.dk>
+Miquel Sabaté Solà <mikisabate@gmail.com>
+Moov Corporation
+Moriyoshi Koizumi <mozo@mozo.jp>
+Môshe van der Sterre <moshevds@gmail.com>
+Nan Deng <monnand@gmail.com>
+Nathan John Youngman <nj@nathany.com>
+ngmoco, LLC
+Nicholas Katsaros <nick@nickkatsaros.com>
+Nicholas Presta <nick@nickpresta.ca> <nick1presta@gmail.com>
+Nicholas Sullivan <nicholas.sullivan@gmail.com>
+Nicholas Waples <nwaples@gmail.com>
+Nick Craig-Wood <nick@craig-wood.com> <nickcw@gmail.com>
+Nicolas Kaiser <nikai@nikai.net>
+Nicolas Owens <mischief@offblast.org>
+Nigel Kerr <nigel.kerr@gmail.com>
+Noah Campbell <noahcampbell@gmail.com>
+Oling Cat <olingcat@gmail.com>
+Oliver Hookins <ohookins@gmail.com>
+Olivier Antoine <olivier.antoine@gmail.com>
+Olivier Duperray <duperray.olivier@gmail.com>
+Olivier Saingre <osaingre@gmail.com>
+Padraig Kitterick <padraigkitterick@gmail.com>
+Paolo Giarrusso <p.giarrusso@gmail.com>
+Pascal S. de Kloe <pascal@quies.net>
+Patrick Crosby <patrick@stathat.com>
+Patrick Gavlin <pgavlin@gmail.com>
+Patrick Higgins <patrick.allen.higgins@gmail.com>
+Patrick Mézard <patrick@mezard.eu>
+Patrick Mylund Nielsen <patrick@patrickmn.com>
+Patrick Smith <pat42smith@gmail.com>
+Paul A Querna <paul.querna@gmail.com>
+Paul Hammond <paul@paulhammond.org>
+Paul Lalonde <paul.a.lalonde@gmail.com>
+Paul Sbarra <Sbarra.Paul@gmail.com>
+Paul van Brouwershaven <paul@vanbrouwershaven.com>
+Pavel Zinovkin <pavel.zinovkin@gmail.com>
+Petar Maymounkov <petarm@gmail.com>
+Peter Armitage <peter.armitage@gmail.com>
+Peter Froehlich <peter.hans.froehlich@gmail.com>
+Peter Kleiweg <pkleiweg@xs4all.nl>
+Peter Mundy <go.peter.90@gmail.com>
+Péter Surányi <speter.go1@gmail.com>
+Péter Szilágyi <peterke@gmail.com>
+Peter Waller <peter.waller@gmail.com>
+Peter Williams <pwil3058@gmail.com>
+Philip K. Warren <pkwarren@gmail.com>
+Pieter Droogendijk <pieter@binky.org.uk>
+Pietro Gagliardi <pietro10@mac.com>
+Preetam Jinka <pj@preet.am>
+Quan Yong Zhai <qyzhai@gmail.com>
+Raif S. Naffah <go@naffah-raif.name>
+Rémy Oudompheng <oudomphe@phare.normalesup.org>
+Richard Crowley <r@rcrowley.org>
+Richard Eric Gavaletz <gavaletz@gmail.com>
+Richard Musiol <mail@richard-musiol.de>
+Rick Arnold <rickarnoldjr@gmail.com>
+Risto Jaakko Saarelma <rsaarelm@gmail.com>
+Robert Daniel Kortschak <dan.kortschak@adelaide.edu.au>
+Robert Dinu <r@varp.se>
+Robert Figueiredo <robfig@gmail.com>
+Robert Hencke <robert.hencke@gmail.com>
+Robert Obryk <robryk@gmail.com>
+Robin Eklind <r.eklind.87@gmail.com>
+Rodrigo Moraes de Oliveira <rodrigo.moraes@gmail.com>
+Rodrigo Rafael Monti Kochenburger <divoxx@gmail.com>
+Roger Pau Monné <royger@gmail.com>
+Roger Peppe <rogpeppe@gmail.com>
+Ron Minnich <rminnich@gmail.com>
+Ross Light <rlight2@gmail.com>
+Rowan Worth <sqweek@gmail.com>
+Ryan Hitchman <hitchmanr@gmail.com>
+Ryan Slade <ryanslade@gmail.com>
+S.Çağlar Onur <caglar@10ur.org>
+Sanjay Menakuru <balasanjay@gmail.com>
+Scott Ferguson <scottwferg@gmail.com>
+Scott Lawrence <bytbox@gmail.com>
+Sebastien Binet	<seb.binet@gmail.com>
+Sébastien Paolacci <sebastien.paolacci@gmail.com>
+Sergei Skorobogatov <skorobo@rambler.ru>
+Sergey 'SnakE'  Gromov <snake.scaly@gmail.com>
+Sergio Luis O. B. Correia <sergio@correia.cc>
+Shane Hansen <shanemhansen@gmail.com>
+Shawn Smith <shawn.p.smith@gmail.com>
+Shenghou Ma <minux.ma@gmail.com>
+Shivakumar GN <shivakumar.gn@gmail.com>
+Sokolov Yura <funny.falcon@gmail.com>
+Spring Mc <heresy.mc@gmail.com>
+StalkR <stalkr@stalkr.net>
+Stefan Nilsson <snilsson@nada.kth.se> <trolleriprofessorn@gmail.com>
+Stéphane Travostino <stephane.travostino@gmail.com>
+Stephen McQuay <stephen@mcquay.me>
+Stephen Weinberg <stephen@q5comm.com>
+Steve McCoy <mccoyst@gmail.com>
+Steven Elliot Harris <seharris@gmail.com>
+Steven Hartland <steven.hartland@multiplay.co.uk>
+Sven Almgren <sven@tras.se>
+Szabolcs Nagy <nsz@port70.net>
+Tad Glines <tad.glines@gmail.com>
+Taj Khattra <taj.khattra@gmail.com>
+Tarmigan Casebolt <tarmigan@gmail.com>
+Taru Karttunen <taruti@taruti.net>
+Thomas Alan Copeland <talan.copeland@gmail.com>
+Thomas Kappler <tkappler@gmail.com>
+Timo Savola <timo.savola@gmail.com>
+Tobias Columbus <tobias.columbus@gmail.com>
+Tor Andersson <tor.andersson@gmail.com>
+Travis Cline <travis.cline@gmail.com>
+Tudor Golubenco <tudor.g@gmail.com>
+Tw <tw19881113@gmail.com>
+Tyler Bunnell <tylerbunnell@gmail.com>
+Ugorji Nwoke <ugorji@gmail.com>
+Ulf Holm Nielsen <doktor@dyregod.dk>
+Uriel Mangado <uriel@berlinblue.org>
+Vadim Vygonets <unixdj@gmail.com>
+Vincent Ambo <tazjin@googlemail.com>
+Vincent Vanackere <vincent.vanackere@gmail.com>
+Vinu Rajashekhar <vinutheraj@gmail.com>
+Vladimir Nikishenko <vova616@gmail.com>
+Volker Dobler <dr.volker.dobler@gmail.com>
+Wei Guangjing <vcc.163@gmail.com>
+Willem van der Schyff <willemvds@gmail.com>
+William Josephson <wjosephson@gmail.com>
+William Orr <will@worrbase.com> <ay1244@gmail.com>
+Xing Xing <mikespook@gmail.com>
+Yasuhiro Matsumoto <mattn.jp@gmail.com>
+Yissakhar Z. Beck <yissakhar.beck@gmail.com>
+Yongjian Xu <i3dmaster@gmail.com>
+Yoshiyuki Kanno <nekotaroh@gmail.com> <yoshiyuki.kanno@stoic.co.jp>
+Yusuke Kagiwada <block.rxckin.beats@gmail.com>
+Yuusei Kuwana <kuwana@kumama.org>
+Yuval Pavel Zholkover <paulzhol@gmail.com>
+Ziad Hatahet <hatahet@gmail.com>
+Zorion Arrizabalaga <zorionk@gmail.com>
+申习之 <bronze1man@gmail.com>
diff --git a/internal/stdlib/testdata/v1.3.2/CONTRIBUTORS b/internal/stdlib/testdata/v1.3.2/CONTRIBUTORS
new file mode 100644
index 0000000..3722298
--- /dev/null
+++ b/internal/stdlib/testdata/v1.3.2/CONTRIBUTORS
@@ -0,0 +1,581 @@
+# This is the official list of people who can contribute
+# (and typically have contributed) code to the Go repository.
+# The AUTHORS file lists the copyright holders; this file
+# lists people.  For example, Google employees are listed here
+# but not in AUTHORS, because Google holds the copyright.
+#
+# The submission process automatically checks to make sure
+# that people submitting code are listed in this file (by email address).
+#
+# Names should be added to this file only after verifying that
+# the individual or the individual's organization has agreed to
+# the appropriate Contributor License Agreement, found here:
+#
+#     http://code.google.com/legal/individual-cla-v1.0.html
+#     http://code.google.com/legal/corporate-cla-v1.0.html
+#
+# The agreement for individuals can be filled out on the web.
+#
+# When adding J Random Contributor's name to this file,
+# either J's name or J's organization's name should be
+# added to the AUTHORS file, depending on whether the
+# individual or corporate CLA was used.
+
+# Names should be added to this file like so:
+#     Name <email address>
+#
+# An entry with two email addresses specifies that the
+# first address should be used in the submit logs and
+# that the second address should be recognized as the
+# same person when interacting with Rietveld.
+
+# Please keep the list sorted.
+
+Aaron France <aaron.l.france@gmail.com>
+Aaron Kemp <kemp.aaron@gmail.com>
+Abhinav Gupta <abhinav.g90@gmail.com>
+Adam Langley <agl@golang.org>
+Adrian Nos <nos.adrian@gmail.com>
+Adrian O'Grady <elpollouk@gmail.com>
+Adrien Bustany <adrien-xx-google@bustany.org>
+Akshat Kumar <seed@mail.nanosouffle.net>
+Alan Donovan <adonovan@google.com>
+Albert Strasheim <fullung@gmail.com>
+Alberto García Hierro <alberto@garciahierro.com> <alberto.garcia.hierro@gmail.com>
+Aleksandar Dezelin <dezelin@gmail.com>
+Alex A Skinner <alex@lx.lc>
+Alex Brainman <alex.brainman@gmail.com>
+Alex Bramley <abramley@google.com>
+Alex Jin <toalexjin@gmail.com>
+Alexander Larsson <alexander.larsson@gmail.com>
+Alexander Orlov <alexander.orlov@loxal.net>
+Alexander Reece <awreece@gmail.com>
+Alexander Surma <surma@surmair.de>
+Alexander Zhavnerchik <alex.vizor@gmail.com>
+Alexandre Normand <alexandre.normand@gmail.com>
+Alexandru Moșoi <brtzsnr@gmail.com>
+Alexei Sholik <alcosholik@gmail.com>
+Alexey Borzenkov <snaury@gmail.com>
+Alexey Palazhchenko <alexey.palazhchenko@gmail.com>
+Alexis Imperial-Legrand <ail@google.com>
+Amir Mohammad Saied <amir@gluegadget.com>
+Amrut Joshi <amrut.joshi@gmail.com>
+Andrea Spadaccini <spadaccio@google.com>
+Andreas Jellinghaus <andreas@ionisiert.de> <anj@google.com>
+Andrei Vieru <euvieru@gmail.com>
+Andrew Balholm <andybalholm@gmail.com>
+Andrew Bonventre <andybons@chromium.org>
+Andrew Gerrand <adg@golang.org>
+Andrew Harding <andrew@spacemonkey.com>
+Andrew Lutomirski <andy@luto.us>
+Andrew Pritchard <awpritchard@gmail.com>
+Andrew Radev <andrey.radev@gmail.com>
+Andrew Skiba <skibaa@gmail.com>
+Andrew Szeto <andrew@jabagawee.com>
+Andrew Wilkins <axwalk@gmail.com>
+Andrey Mirtchovski <mirtchovski@gmail.com>
+Andriy Lytvynov <lytvynov.a.v@gmail.com>
+Andy Davis <andy@bigandian.com>
+Anfernee Yongkun Gui <anfernee.gui@gmail.com>
+Anh Hai Trinh <anh.hai.trinh@gmail.com>
+Anschel Schaffer-Cohen <anschelsc@gmail.com>
+Anthony Eufemio <anthony.eufemio@gmail.com>
+Anthony Martin <ality@pbrane.org>
+Anthony Starks <ajstarks@gmail.com>
+Apisak Darakananda <pongad@gmail.com>
+Aram Hăvărneanu <aram@mgk.ro>
+Arnaud Ysmal <arnaud.ysmal@gmail.com>
+Arne Hormann <arnehormann@gmail.com>
+Aron Nopanen <aron.nopanen@gmail.com>
+Arvindh Rajesh Tamilmani <art@a-30.net>
+Asim Shankar <asimshankar@gmail.com>
+Ato Araki <ato.araki@gmail.com>
+Aulus Egnatius Varialus <varialus@gmail.com>
+Austin Clements <aclements@csail.mit.edu>
+Balazs Lecz <leczb@google.com>
+Ben Eitzen <eitzenb@golang.org>
+Ben Fried <ben.fried@gmail.com>
+Ben Lynn <benlynn@gmail.com>
+Ben Olive <sionide21@gmail.com>
+Benjamin Black <b@b3k.us>
+Benny Siegert <bsiegert@gmail.com>
+Berengar Lehr <Berengar.Lehr@gmx.de>
+Bill Neubauer <wcn@golang.org> <wcn@google.com> <bill.neubauer@gmail.com>
+Bill Thiede <couchmoney@gmail.com>
+Billie Harold Cleek <bhcleek@gmail.com>
+Bjorn Tillenius <bjorn@tillenius.me>
+Bjorn Tipling <bjorn.tipling@gmail.com>
+Blake Mizerany <blake.mizerany@gmail.com>
+Bobby Powers <bobbypowers@gmail.com>
+Brad Fitzpatrick <bradfitz@golang.org> <bradfitz@gmail.com>
+Brad Garcia <bgarcia@golang.org>
+Brendan Daniel Tracey <tracey.brendan@gmail.com>
+Brendan O'Dea <bod@golang.org>
+Brian Dellisanti <briandellisanti@gmail.com>
+Brian G. Merrell <bgmerrell@gmail.com>
+Brian Gitonga Marete <marete@toshnix.com>
+Brian Ketelsen <bketelsen@gmail.com>
+Brian Slesinsky <skybrian@google.com>
+Burcu Dogan <jbd@google.com>
+Caine Tighe <arctanofyourface@gmail.com>
+Caleb Spare <cespare@gmail.com>
+Carl Chatfield <carlchatfield@gmail.com>
+Carl Mastrangelo <notcarl@google.com>
+Carl Shapiro <cshapiro@google.com> <cshapiro@golang.org>
+Carlos Castillo <cookieo9@gmail.com>
+Cary Hull <chull@google.com>
+Case Nelson <case.nelson@gmail.com>
+Casey Marshall <casey.marshall@gmail.com>
+Catalin Patulea <catalinp@google.com>
+Cezar Sá Espinola <cezarsa@gmail.com>
+ChaiShushan <chaishushan@gmail.com>
+Charles L. Dorian <cldorian@gmail.com>
+Charles Lee <zombie.fml@gmail.com>
+Chris Dollin <ehog.hedge@gmail.com>
+Chris Farmiloe <chrisfarms@gmail.com>
+Chris Howey <howeyc@gmail.com>
+Chris Hundt <hundt@google.com>
+Chris Jones <chris@cjones.org> <chris.jones.yar@gmail.com>
+Chris Lennert <calennert@gmail.com>
+Chris Manghane <cmang@golang.org>
+Christian Himpel <chressie@googlemail.com> <chressie@gmail.com>
+Christine Hansmann <chhansmann@gmail.com>
+Christoffer Buchholz <christoffer.buchholz@gmail.com>
+Christoph Hack <christoph@tux21b.org>
+Christopher Cahoon <chris.cahoon@gmail.com>
+Christopher Nielsen <m4dh4tt3r@gmail.com>
+Christopher Redden <christopher.redden@gmail.com>
+Christopher Swenson <cswenson@google.com>
+Christopher Wedgwood <cw@f00f.org>
+Clement Skau <clementskau@gmail.com>
+Colby Ranger <cranger@google.com>
+Conrad Meyer <cemeyer@cs.washington.edu>
+Corey Thomasson <cthom.lists@gmail.com>
+Cosmos Nicolaou <cnicolaou@google.com>
+Damian Gryski <dgryski@gmail.com>
+Dan Callahan <dan.callahan@gmail.com>
+Dan Peterson <dpiddy@gmail.com>
+Dan Sinclair <dan.sinclair@gmail.com>
+Daniel Fleischman <danielfleischman@gmail.com>
+Daniel Krech <eikeon@eikeon.com>
+Daniel Lidén <daniel.liden.87@gmail.com>
+Daniel Morsing <daniel.morsing@gmail.com>
+Daniel Nadasi <dnadasi@google.com>
+Daniel Theophanes <kardianos@gmail.com>
+Darren Elwood <darren@textnode.com>
+Dave Borowitz <dborowitz@google.com>
+Dave Cheney <dave@cheney.net>
+Dave Day <djd@golang.org>
+Dave Grijalva <dgrijalva@ngmoco.com>
+David Anderson <danderson@google.com>
+David Barnett <dbarnett@google.com>
+David Bürgin <676c7473@gmail.com>
+David Calavera <david.calavera@gmail.com>
+David Covert <davidhcovert@gmail.com>
+David Crawshaw <david.crawshaw@zentus.com> <crawshaw@google.com> <crawshaw@golang.org>
+David du Colombier <0intro@gmail.com>
+David Forsythe <dforsythe@gmail.com>
+David G. Andersen <dave.andersen@gmail.com>
+David Jakob Fritz <david.jakob.fritz@gmail.com>
+David McLeish <davemc@google.com>
+David Presotto <presotto@gmail.com>
+David Symonds <dsymonds@golang.org>
+David Thomas <davidthomas426@gmail.com>
+David Titarenco <david.titarenco@gmail.com>
+Dean Prichard <dean.prichard@gmail.com>
+Denis Brandolini <denis.brandolini@gmail.com>
+Devon H. O'Dell <devon.odell@gmail.com>
+Dhiru Kholia <dhiru.kholia@gmail.com>
+Dimitri Tcaciuc <dtcaciuc@gmail.com>
+Dmitri Shuralyov <shurcooL@gmail.com>
+Dmitriy Shelenin <deemok@googlemail.com> <deemok@gmail.com>
+Dmitriy Vyukov <dvyukov@google.com>
+Dmitry Chestnykh <dchest@gmail.com>
+Dominik Honnef <dominik.honnef@gmail.com>
+Donovan Hide <donovanhide@gmail.com>
+Drew Hintz <adhintz@google.com>
+Duncan Holm <mail@frou.org>
+Dustin Sallings <dsallings@gmail.com>
+Dustin Shields-Cloues <dcloues@gmail.com>
+Eden Li <eden.li@gmail.com>
+Egon Elbre <egonelbre@gmail.com>
+Ehren Kret <ehren.kret@gmail.com>
+Eivind Uggedal <eivind@uggedal.com>
+Elias Naur <elias.naur@gmail.com>
+Emil Hessman <c.emil.hessman@gmail.com>
+Eoghan Sherry <ejsherry@gmail.com>
+Eric Clark <zerohp@gmail.com>
+Eric Milliken <emilliken@gmail.com>
+Eric Roshan-Eisner <eric.d.eisner@gmail.com>
+Erik St. Martin <alakriti@gmail.com>
+Erik Westrup <erik.westrup@gmail.com>
+Esko Luontola <esko.luontola@gmail.com>
+Evan Martin <evan.martin@gmail.com>
+Evan Shaw <chickencha@gmail.com>
+Ewan Chou <coocood@gmail.com>
+Fabrizio Milo <mistobaan@gmail.com>
+Fan Hongjian <fan.howard@gmail.com>
+Fazlul Shahriar <fshahriar@gmail.com>
+Felix Geisendörfer <haimuiba@gmail.com>
+Firmansyah Adiputra <frm.adiputra@gmail.com>
+Florian Uekermann <florian@uekermann-online.de> <f1@uekermann-online.de>
+Florian Weimer <fw@deneb.enyo.de>
+Folke Behrens <folke@google.com>
+Francesc Campoy <campoy@golang.org>
+Francisco Souza <franciscossouza@gmail.com>
+Frederick Kelly Mayle III <frederickmayle@gmail.com>
+Fredrik Enestad <fredrik.enestad@soundtrackyourbrand.com>
+Frithjof Schulze <schulze@math.uni-hannover.de> <sfrithjof@gmail.com>
+Fumitoshi Ukai <ukai@google.com>
+Gaal Yahas <gaal@google.com>
+Gary Burd <gary@beagledreams.com> <gary.burd@gmail.com>
+Gautham Thambidorai <gautham.dorai@gmail.com>
+Georg Reinke <guelfey@gmail.com>
+Gerasimos Dimitriadis <gedimitr@gmail.com>
+Gideon Jan-Wessel Redelinghuys <gjredelinghuys@gmail.com>
+Giles Lean <giles.lean@pobox.com>
+Gordon Klaus <gordon.klaus@gmail.com>
+Graham King <graham4king@gmail.com>
+Graham Miller <graham.miller@gmail.com>
+Greg Ward <greg@gerg.ca>
+Guillaume J. Charmes <guillaume@charmes.net>
+Gustav Paul <gustav.paul@gmail.com>
+Gustavo Franco <gustavorfranco@gmail.com>
+Gustavo Niemeyer <gustavo@niemeyer.net> <n13m3y3r@gmail.com>
+Gwenael Treguier <gwenn.kahz@gmail.com>
+Han-Wen Nienhuys <hanwen@google.com>
+Harley Laue <losinggeneration@gmail.com>
+Hector Chu <hectorchu@gmail.com>
+Henrik Edwards <henrik.edwards@gmail.com>
+Herbert Georg Fischer <herbert.fischer@gmail.com>
+Hong Ruiqi <hongruiqi@gmail.com>
+Hossein Sheikh Attar <hattar@google.com>
+Ian Lance Taylor <iant@golang.org>
+Icarus Sparry <golang@icarus.freeuk.com>
+Ingo Oeser <nightlyone@googlemail.com> <nightlyone@gmail.com>
+Isaac Wagner <ibw@isaacwagner.me>
+Ivan Krasin <krasin@golang.org>
+Jacob Baskin <jbaskin@google.com>
+Jakob Borg <jakob@nym.se>
+Jakub Ryszard Czarnowicz <j.czarnowicz@gmail.com>
+James Aguilar <jaguilar@google.com>
+James David Chalfant <james.chalfant@gmail.com>
+James Fysh <james.fysh@gmail.com>
+James Gray <james@james4k.com>
+James Meneghello <rawrz0r@gmail.com>
+James P. Cooper <jamespcooper@gmail.com>
+James Toy <nil@opensesame.st>
+James Tucker <raggi@google.com>
+James Whitehead <jnwhiteh@gmail.com>
+Jamie Gennis <jgennis@google.com> <jgennis@gmail.com>
+Jamie Turner <jamwt@dropbox.com>
+Jamie Wilkinson <jaq@spacepants.org>
+Jan H. Hosang <jan.hosang@gmail.com>
+Jan Mercl <0xjnml@gmail.com>
+Jan Mercl <befelemepeseveze@gmail.com>
+Jan Newmarch <jan.newmarch@gmail.com>
+Jan Ziak <0xe2.0x9a.0x9b@gmail.com>
+Jani Monoses <jani.monoses@ubuntu.com> <jani.monoses@gmail.com>
+Jaroslavas Počepko <jp@webmaster.ms>
+Jason Del Ponte <delpontej@gmail.com>
+Jason Travis <infomaniac7@gmail.com>
+Jay Weisskopf <jay@jayschwa.net>
+Jean-Marc Eurin <jmeurin@google.com>
+Jeff Hodges <jeff@somethingsimilar.com>
+Jeff R. Allen <jra@nella.org> <jeff.allen@gmail.com>
+Jeff Sickel <jas@corpus-callosum.com>
+Jeff Wendling <jeff@spacemonkey.com>
+Jeremiah Harmsen <jeremiah@google.com>
+Jeremy Jackins <jeremyjackins@gmail.com>
+Jeremy Schlatter <jeremy.schlatter@gmail.com>
+Jim McGrath <jimmc2@gmail.com>
+Jimmy Zelinskie <jimmyzelinskie@gmail.com>
+Jingcheng Zhang <diogin@gmail.com>
+Joakim Sernbrant <serbaut@gmail.com>
+Joe Poirier <jdpoirier@gmail.com>
+Joel Sing <jsing@google.com>
+Johan Euphrosine <proppy@google.com>
+John Asmuth <jasmuth@gmail.com>
+John Beisley <huin@google.com>
+John C Barstow <jbowtie@amathaine.com>
+John DeNero <denero@google.com>
+John Graham-Cumming <jgc@jgc.org> <jgrahamc@gmail.com>
+John Howard Palevich <jack.palevich@gmail.com>
+John Newlin <jnewlin@google.com>
+John Shahid <jvshahid@gmail.com>
+Jonathan Allie <jonallie@google.com>
+Jonathan Feinberg <feinberg@google.com>
+Jonathan Gold <jgold.bg@gmail.com>
+Jonathan Hseu <jhseu@google.com>
+Jonathan Mark <jhmark@xenops.com> <jhmark000@gmail.com>
+Jonathan Nieder <jrn@google.com>
+Jonathan Pittman <jmpittman@google.com> <jonathan.mark.pittman@gmail.com>
+Jonathan Rudenberg <jonathan@titanous.com>
+Jonathan Wills <runningwild@gmail.com>
+Jongmin Kim <atomaths@gmail.com>
+Jos Visser <josv@google.com>
+Jose Luis Vázquez González <josvazg@gmail.com>
+Joseph Bonneau <jcb@google.com>
+Joseph Holsten <joseph@josephholsten.com>
+Josh Bleecher Snyder <josharian@gmail.com>
+Josh Goebel <dreamer3@gmail.com>
+Josh Hoak <jhoak@google.com>
+Josh Holland <jrh@joshh.co.uk>
+Joshua Chase <jcjoshuachase@gmail.com>
+JP Sugarbroad <jpsugar@google.com>
+Jukka-Pekka Kekkonen <karatepekka@gmail.com>
+Julian Phillips <julian@quantumfyre.co.uk>
+Julien Schmidt <google@julienschmidt.com>
+Kai Backman <kaib@golang.org>
+Kamil Kisiel <kamil@kamilkisiel.net> <kamil.kisiel@gmail.com>
+Katrina Owen <katrina.owen@gmail.com>
+Kay Zhu <kayzhu@google.com>
+Kei Son <hey.calmdown@gmail.com>
+Keith Randall <khr@golang.org>
+Keith Rarick <kr@xph.us>
+Kelsey Hightower <kelsey.hightower@gmail.com>
+Kelvin Foo Chuan Lyi <vmirage@gmail.com>
+Ken Friedenbach <kenliz@cruzio.com>
+Ken Rockot <ken@oz.gs> <ken.rockot@gmail.com>
+Ken Thompson <ken@golang.org>
+Kevin Ballard <kevin@sb.org>
+Kevin Klues <klueska@gmail.com> <klueska@google.com>
+Kirklin McDonald <kirklin.mcdonald@gmail.com>
+Kyle Consalus <consalus@gmail.com>
+Kyle Isom <kyle@gokyle.net>
+Kyle Lemons <kyle@kylelemons.net> <kevlar@google.com>
+L Campbell <unpantsu@gmail.com>
+Lai Jiangshan <eag0628@gmail.com>
+Larry Hosken <lahosken@golang.org>
+Lorenzo Stoakes <lstoakes@gmail.com>
+Louis Kruger <louisk@google.com>
+Luca Greco <luca.greco@alcacoop.it>
+Lucio De Re <lucio.dere@gmail.com>
+Luit van Drongelen <luitvd@gmail.com>
+Luka Zakrajšek <tr00.g33k@gmail.com>
+Luke Curley <qpingu@gmail.com>
+Luuk van Dijk <lvd@golang.org> <lvd@google.com>
+Manoj Dayaram <platform-dev@moovweb.com> <manoj.dayaram@moovweb.com>
+Manu Garg <manugarg@google.com>
+Marc Weistroff <marc@weistroff.net>
+Marcel van Lohuizen <mpvl@golang.org>
+Marco Hennings <marco.hennings@freiheit.com>
+Mark Zavislak <zavislak@google.com>
+Marko Juhani Silokunnas <marko.silokunnas@gmail.com>
+Marko Mikulicic <mkm@google.com>
+Marko Tiikkaja <marko@joh.to>
+Markus Duft <markus.duft@salomon.at>
+Markus Sonderegger <marraison@gmail.com>
+Markus Zimmermann <zimmski@gmail.com>
+Martin Neubauer <m.ne@gmx.net>
+Martin Olsson <martin@minimum.se>
+Mateusz Czapliński <czapkofan@gmail.com>
+Mathieu Lonjaret <mathieu.lonjaret@gmail.com>
+Mats Lidell <mats.lidell@cag.se> <mats.lidell@gmail.com>
+Matt Aimonetti <mattaimonetti@gmail.com>
+Matt Brown <mdbrown@google.com>
+Matt Jibson <matt.jibson@gmail.com>
+Matt Joiner <anacrolix@gmail.com>
+Matt Jones <mrjones@google.com>
+Matt Reiferson <mreiferson@gmail.com>
+Matthew Cottingham <mattcottingham@gmail.com>
+Matthew Dempsky <mdempsky@google.com>
+Matthew Horsnell <matthew.horsnell@gmail.com>
+Maxim Khitrov <max@mxcrypt.com>
+Maxim Pimenov <mpimenov@google.com>
+Maxim Ushakov <ushakov@google.com>
+Micah Stetson <micah.stetson@gmail.com>
+Michael Chaten <mchaten@gmail.com>
+Michael Elkins <michael.elkins@gmail.com>
+Michael Fraenkel <michael.fraenkel@gmail.com>
+Michael Gehring <mg@ebfe.org> <gnirheg.leahcim@gmail.com>
+Michael Hoisie <hoisie@gmail.com>
+Michael Hudson-Doyle <michael.hudson@linaro.org>
+Michael Kelly <mjk@google.com>
+Michael Lewis <mikelikespie@gmail.com>
+Michael Matloob <matloob@google.com>
+Michael Pearson <mipearson@gmail.com>
+Michael Piatek <piatek@google.com>
+Michael Shields <mshields@google.com>
+Michael Stapelberg <michael@stapelberg.de> <mstplbrg@googlemail.com>
+Michael T. Jones <mtj@google.com> <michael.jones@gmail.com>
+Michael Teichgräber <mteichgraeber@gmx.de> <mt4swm@googlemail.com>
+Michał Derkacz <ziutek@lnet.pl>
+Miek Gieben <miek@miek.nl> <remigius.gieben@gmail.com>
+Mihai Borobocea <MihaiBorobocea@gmail.com>
+Mikael Tillenius <mikti42@gmail.com>
+Mike Andrews <mra@xoba.com>
+Mike Rosset <mike.rosset@gmail.com>
+Mike Samuel <mikesamuel@gmail.com>
+Mike Solomon <msolo@gmail.com>
+Mikhail Panchenko <m@mihasya.com>
+Miki Tebeka <miki.tebeka@gmail.com>
+Mikio Hara <mikioh.mikioh@gmail.com>
+Mikkel Krautz <mikkel@krautz.dk> <krautz@gmail.com>
+Miquel Sabaté Solà <mikisabate@gmail.com>
+Moriyoshi Koizumi <mozo@mozo.jp>
+Môshe van der Sterre <moshevds@gmail.com>
+Nan Deng <monnand@gmail.com>
+Nathan John Youngman <nj@nathany.com>
+Nicholas Katsaros <nick@nickkatsaros.com>
+Nicholas Presta <nick@nickpresta.ca> <nick1presta@gmail.com>
+Nicholas Sullivan <nicholas.sullivan@gmail.com>
+Nicholas Waples <nwaples@gmail.com>
+Nick Craig-Wood <nick@craig-wood.com> <nickcw@gmail.com>
+Nicolas Kaiser <nikai@nikai.net>
+Nicolas Owens <mischief@offblast.org>
+Nigel Kerr <nigel.kerr@gmail.com>
+Nigel Tao <nigeltao@golang.org>
+Noah Campbell <noahcampbell@gmail.com>
+Oling Cat <olingcat@gmail.com>
+Oliver Hookins <ohookins@gmail.com>
+Olivier Antoine <olivier.antoine@gmail.com>
+Olivier Duperray <duperray.olivier@gmail.com>
+Olivier Saingre <osaingre@gmail.com>
+Padraig Kitterick <padraigkitterick@gmail.com>
+Paolo Giarrusso <p.giarrusso@gmail.com>
+Pascal S. de Kloe <pascal@quies.net>
+Patrick Crosby <patrick@stathat.com>
+Patrick Gavlin <pgavlin@gmail.com>
+Patrick Higgins <patrick.allen.higgins@gmail.com>
+Patrick Mézard <patrick@mezard.eu>
+Patrick Mylund Nielsen <patrick@patrickmn.com>
+Patrick Riley <pfr@google.com>
+Patrick Smith <pat42smith@gmail.com>
+Paul A Querna <paul.querna@gmail.com>
+Paul Borman <borman@google.com>
+Paul Chang <paulchang@google.com>
+Paul Hammond <paul@paulhammond.org>
+Paul Lalonde <paul.a.lalonde@gmail.com>
+Paul Sbarra <Sbarra.Paul@gmail.com>
+Paul van Brouwershaven <paul@vanbrouwershaven.com>
+Pavel Zinovkin <pavel.zinovkin@gmail.com>
+Pawel Szczur <filemon@google.com>
+Petar Maymounkov <petarm@gmail.com>
+Peter Armitage <peter.armitage@gmail.com>
+Peter Collingbourne <pcc@google.com>
+Peter Froehlich <peter.hans.froehlich@gmail.com>
+Peter Kleiweg <pkleiweg@xs4all.nl>
+Peter McKenzie <petermck@google.com>
+Peter Mundy <go.peter.90@gmail.com>
+Péter Surányi <speter.go1@gmail.com>
+Péter Szabó <pts@google.com>
+Péter Szilágyi <peterke@gmail.com>
+Peter Waller <peter.waller@gmail.com>
+Peter Weinberger <pjw@golang.org>
+Peter Williams <pwil3058@gmail.com>
+Phil Pennock <pdp@golang.org>
+Philip K. Warren <pkwarren@gmail.com>
+Pieter Droogendijk <pieter@binky.org.uk>
+Pietro Gagliardi <pietro10@mac.com>
+Preetam Jinka <pj@preet.am>
+Quan Yong Zhai <qyzhai@gmail.com>
+Raif S. Naffah <go@naffah-raif.name>
+Raph Levien <raph@google.com>
+Rémy Oudompheng <oudomphe@phare.normalesup.org> <remyoudompheng@gmail.com>
+Richard Crowley <r@rcrowley.org>
+Richard Eric Gavaletz <gavaletz@gmail.com>
+Richard Musiol <mail@richard-musiol.de> <neelance@gmail.com>
+Rick Arnold <rickarnoldjr@gmail.com>
+Risto Jaakko Saarelma <rsaarelm@gmail.com>
+Rob Pike <r@golang.org>
+Robert Daniel Kortschak <dan.kortschak@adelaide.edu.au>
+Robert Dinu <r@varp.se>
+Robert Figueiredo <robfig@gmail.com>
+Robert Griesemer <gri@golang.org>
+Robert Hencke <robert.hencke@gmail.com>
+Robert Obryk <robryk@gmail.com>
+Robert Sesek <rsesek@google.com>
+Robert Snedegar <roberts@google.com>
+Robin Eklind <r.eklind.87@gmail.com>
+Rodrigo Moraes de Oliveira <rodrigo.moraes@gmail.com>
+Rodrigo Rafael Monti Kochenburger <divoxx@gmail.com>
+Roger Pau Monné <royger@gmail.com>
+Roger Peppe <rogpeppe@gmail.com>
+Ron Minnich <rminnich@gmail.com>
+Ross Light <rlight2@gmail.com>
+Rowan Worth <sqweek@gmail.com>
+Rui Ueyama <ruiu@google.com>
+Russ Cox <rsc@golang.org>
+Ryan Barrett <ryanb@google.com>
+Ryan Hitchman <hitchmanr@gmail.com>
+Ryan Slade <ryanslade@gmail.com>
+S.Çağlar Onur <caglar@10ur.org>
+Sam Thorogood <thorogood@google.com> <sam.thorogood@gmail.com>
+Sameer Ajmani <sameer@golang.org> <ajmani@gmail.com>
+Sanjay Menakuru <balasanjay@gmail.com>
+Scott Ferguson <scottwferg@gmail.com>
+Scott Lawrence <bytbox@gmail.com>
+Scott Schwartz <scotts@golang.org>
+Sean Burford <sburford@google.com>
+Sebastien Binet	<seb.binet@gmail.com>
+Sébastien Paolacci <sebastien.paolacci@gmail.com>
+Sergei Skorobogatov <skorobo@rambler.ru>
+Sergey 'SnakE' Gromov <snake.scaly@gmail.com>
+Sergio Luis O. B. Correia <sergio@correia.cc>
+Shane Hansen <shanemhansen@gmail.com>
+Shawn Ledbetter <sledbetter@google.com>
+Shawn Smith <shawn.p.smith@gmail.com>
+Shenghou Ma <minux@golang.org> <minux.ma@gmail.com>
+Shivakumar GN <shivakumar.gn@gmail.com>
+Sokolov Yura <funny.falcon@gmail.com>
+Spring Mc <heresy.mc@gmail.com>
+StalkR <stalkr@stalkr.net>
+Stefan Nilsson <snilsson@nada.kth.se> <trolleriprofessorn@gmail.com>
+Stéphane Travostino <stephane.travostino@gmail.com>
+Stephen Ma <stephenm@golang.org>
+Stephen McQuay <stephen@mcquay.me>
+Stephen Weinberg <stephen@q5comm.com>
+Steve McCoy <mccoyst@gmail.com>
+Steven Elliot Harris <seharris@gmail.com>
+Steven Hartland <steven.hartland@multiplay.co.uk>
+Sugu Sougoumarane <ssougou@gmail.com>
+Sven Almgren <sven@tras.se>
+Szabolcs Nagy <nsz@port70.net>
+Tad Glines <tad.glines@gmail.com>
+Taj Khattra <taj.khattra@gmail.com>
+Tarmigan Casebolt <tarmigan@gmail.com>
+Taru Karttunen <taruti@taruti.net>
+Thomas Alan Copeland <talan.copeland@gmail.com>
+Thomas Habets <habets@google.com>
+Thomas Kappler <tkappler@gmail.com>
+Timo Savola <timo.savola@gmail.com>
+Tobias Columbus <tobias.columbus@gmail.com> <tobias.columbus@googlemail.com>
+Todd Wang <toddwang@gmail.com>
+Tom Szymanski <tgs@google.com>
+Tor Andersson <tor.andersson@gmail.com>
+Travis Cline <travis.cline@gmail.com>
+Trevor Strohman <trevor.strohman@gmail.com>
+Tudor Golubenco <tudor.g@gmail.com>
+Tw <tw19881113@gmail.com>
+Tyler Bunnell <tylerbunnell@gmail.com>
+Ugorji Nwoke <ugorji@gmail.com>
+Ulf Holm Nielsen <doktor@dyregod.dk>
+Uriel Mangado <uriel@berlinblue.org>
+Vadim Vygonets <unixdj@gmail.com>
+Vega Garcia Luis Alfonso <vegacom@gmail.com>
+Vincent Ambo <tazjin@googlemail.com>
+Vincent Vanackere <vincent.vanackere@gmail.com>
+Vinu Rajashekhar <vinutheraj@gmail.com>
+Vish Subramanian <vish@google.com>
+Vladimir Nikishenko <vova616@gmail.com>
+Volker Dobler <dr.volker.dobler@gmail.com>
+Wei Guangjing <vcc.163@gmail.com>
+Will Norris <willnorris@google.com>
+Willem van der Schyff <willemvds@gmail.com>
+William Chan <willchan@chromium.org>
+William Josephson <wjosephson@gmail.com>
+William Orr <will@worrbase.com> <ay1244@gmail.com>
+Xing Xing <mikespook@gmail.com>
+Yan Zou <yzou@google.com>
+Yasuhiro Matsumoto <mattn.jp@gmail.com>
+Yissakhar Z. Beck <yissakhar.beck@gmail.com>
+Yongjian Xu <i3dmaster@gmail.com>
+Yoshiyuki Kanno <nekotaroh@gmail.com> <yoshiyuki.kanno@stoic.co.jp>
+Yusuke Kagiwada <block.rxckin.beats@gmail.com>
+Yuusei Kuwana <kuwana@kumama.org>
+Yuval Pavel Zholkover <paulzhol@gmail.com>
+Yves Junqueira <yves.junqueira@gmail.com>
+Ziad Hatahet <hatahet@gmail.com>
+Zorion Arrizabalaga <zorionk@gmail.com>
+申习之 <bronze1man@gmail.com>
diff --git a/internal/stdlib/testdata/v1.3.2/LICENSE b/internal/stdlib/testdata/v1.3.2/LICENSE
new file mode 100644
index 0000000..7448756
--- /dev/null
+++ b/internal/stdlib/testdata/v1.3.2/LICENSE
@@ -0,0 +1,27 @@
+Copyright (c) 2012 The Go Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+   * Redistributions of source code must retain the above copyright
+notice, this list of conditions and the following disclaimer.
+   * Redistributions in binary form must reproduce the above
+copyright notice, this list of conditions and the following disclaimer
+in the documentation and/or other materials provided with the
+distribution.
+   * Neither the name of Google Inc. nor the names of its
+contributors may be used to endorse or promote products derived from
+this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/internal/stdlib/testdata/v1.3.2/PATENTS b/internal/stdlib/testdata/v1.3.2/PATENTS
new file mode 100644
index 0000000..7330990
--- /dev/null
+++ b/internal/stdlib/testdata/v1.3.2/PATENTS
@@ -0,0 +1,22 @@
+Additional IP Rights Grant (Patents)
+
+"This implementation" means the copyrightable works distributed by
+Google as part of the Go project.
+
+Google hereby grants to You a perpetual, worldwide, non-exclusive,
+no-charge, royalty-free, irrevocable (except as stated in this section)
+patent license to make, have made, use, offer to sell, sell, import,
+transfer and otherwise run, modify and propagate the contents of this
+implementation of Go, where such license applies only to those patent
+claims, both currently owned or controlled by Google and acquired in
+the future, licensable by Google that are necessarily infringed by this
+implementation of Go.  This grant does not include claims that would be
+infringed only as a consequence of further modification of this
+implementation.  If you or your agent or exclusive licensee institute or
+order or agree to the institution of patent litigation against any
+entity (including a cross-claim or counterclaim in a lawsuit) alleging
+that this implementation of Go or any code incorporated within this
+implementation of Go constitutes direct or contributory patent
+infringement, or inducement of patent infringement, then any patent
+rights granted to you under this License for this implementation of Go
+shall terminate as of the date such litigation is filed.
diff --git a/internal/stdlib/testdata/v1.3.2/README b/internal/stdlib/testdata/v1.3.2/README
new file mode 100644
index 0000000..a557fe9
--- /dev/null
+++ b/internal/stdlib/testdata/v1.3.2/README
@@ -0,0 +1,32 @@
+This is the source code repository for the Go programming language.  
+
+For documentation about how to install and use Go,
+visit http://golang.org/ or load doc/install-source.html
+in your web browser.
+
+After installing Go, you can view a nicely formatted
+doc/install-source.html by running godoc --http=:6060
+and then visiting http://localhost:6060/doc/install/source.
+
+Unless otherwise noted, the Go source files are distributed
+under the BSD-style license found in the LICENSE file.
+
+--
+
+Binary Distribution Notes
+
+If you have just untarred a binary Go distribution, you need to set
+the environment variable $GOROOT to the full path of the go
+directory (the one containing this README).  You can omit the
+variable if you unpack it into /usr/local/go, or if you rebuild
+from sources by running all.bash (see doc/install.html).
+You should also add the Go binary directory $GOROOT/bin
+to your shell's path.
+
+For example, if you extracted the tar file into $HOME/go, you might
+put the following in your .profile:
+
+    export GOROOT=$HOME/go
+    export PATH=$PATH:$GOROOT/bin
+
+See doc/install.html for more details.
diff --git a/internal/stdlib/testdata/v1.3.2/VERSION b/internal/stdlib/testdata/v1.3.2/VERSION
new file mode 100644
index 0000000..db938eb
--- /dev/null
+++ b/internal/stdlib/testdata/v1.3.2/VERSION
@@ -0,0 +1 @@
+go1.3.2
\ No newline at end of file
diff --git a/internal/stdlib/testdata/v1.3.2/VERSION.cache b/internal/stdlib/testdata/v1.3.2/VERSION.cache
new file mode 100644
index 0000000..9588aef
--- /dev/null
+++ b/internal/stdlib/testdata/v1.3.2/VERSION.cache
@@ -0,0 +1 @@
+devel +a626dacd1c Sat Aug 24 19:56:37 2019 -0400
\ No newline at end of file
diff --git a/internal/stdlib/testdata/v1.3.2/favicon.ico b/internal/stdlib/testdata/v1.3.2/favicon.ico
new file mode 100644
index 0000000..d287722
--- /dev/null
+++ b/internal/stdlib/testdata/v1.3.2/favicon.ico
Binary files differ
diff --git a/internal/stdlib/testdata/v1.3.2/robots.txt b/internal/stdlib/testdata/v1.3.2/robots.txt
new file mode 100644
index 0000000..1f53798
--- /dev/null
+++ b/internal/stdlib/testdata/v1.3.2/robots.txt
@@ -0,0 +1,2 @@
+User-agent: *
+Disallow: /
diff --git a/internal/stdlib/testdata/v1.3.2/src/pkg/errors/errors.go b/internal/stdlib/testdata/v1.3.2/src/pkg/errors/errors.go
new file mode 100644
index 0000000..b8a4692
--- /dev/null
+++ b/internal/stdlib/testdata/v1.3.2/src/pkg/errors/errors.go
@@ -0,0 +1,20 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package errors implements functions to manipulate errors.
+package errors
+
+// New returns an error that formats as the given text.
+func New(text string) error {
+	return &errorString{text}
+}
+
+// errorString is a trivial implementation of error.
+type errorString struct {
+	s string
+}
+
+func (e *errorString) Error() string {
+	return e.s
+}
diff --git a/internal/stdlib/testdata/v1.3.2/src/pkg/errors/errors_test.go b/internal/stdlib/testdata/v1.3.2/src/pkg/errors/errors_test.go
new file mode 100644
index 0000000..cf4df90
--- /dev/null
+++ b/internal/stdlib/testdata/v1.3.2/src/pkg/errors/errors_test.go
@@ -0,0 +1,53 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errors_test
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+func TestNewEqual(t *testing.T) {
+	// Different allocations should not be equal.
+	if errors.New("abc") == errors.New("abc") {
+		t.Errorf(`New("abc") == New("abc")`)
+	}
+	if errors.New("abc") == errors.New("xyz") {
+		t.Errorf(`New("abc") == New("xyz")`)
+	}
+
+	// Same allocation should be equal to itself (not crash).
+	err := errors.New("jkl")
+	if err != err {
+		t.Errorf(`err != err`)
+	}
+}
+
+func TestErrorMethod(t *testing.T) {
+	err := errors.New("abc")
+	if err.Error() != "abc" {
+		t.Errorf(`New("abc").Error() = %q, want %q`, err.Error(), "abc")
+	}
+}
+
+func ExampleNew() {
+	err := errors.New("emit macho dwarf: elf header corrupted")
+	if err != nil {
+		fmt.Print(err)
+	}
+	// Output: emit macho dwarf: elf header corrupted
+}
+
+// The fmt package's Errorf function lets us use the package's formatting
+// features to create descriptive error messages.
+func ExampleNew_errorf() {
+	const name, id = "bimmler", 17
+	err := fmt.Errorf("user %q (id %d) not found", name, id)
+	if err != nil {
+		fmt.Print(err)
+	}
+	// Output: user "bimmler" (id 17) not found
+}
diff --git a/internal/stdlib/testdata/v1.3.2/src/pkg/errors/example_test.go b/internal/stdlib/testdata/v1.3.2/src/pkg/errors/example_test.go
new file mode 100644
index 0000000..5dc8841
--- /dev/null
+++ b/internal/stdlib/testdata/v1.3.2/src/pkg/errors/example_test.go
@@ -0,0 +1,34 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errors_test
+
+import (
+	"fmt"
+	"time"
+)
+
+// MyError is an error implementation that includes a time and message.
+type MyError struct {
+	When time.Time
+	What string
+}
+
+func (e MyError) Error() string {
+	return fmt.Sprintf("%v: %v", e.When, e.What)
+}
+
+func oops() error {
+	return MyError{
+		time.Date(1989, 3, 15, 22, 30, 0, 0, time.UTC),
+		"the file system has gone away",
+	}
+}
+
+func Example() {
+	if err := oops(); err != nil {
+		fmt.Println(err)
+	}
+	// Output: 1989-03-15 22:30:00 +0000 UTC: the file system has gone away
+}