all: rely on dynamic dependency check for genproto

The genproto module incurs an unfortunate amount of dependency bloat
since it has a relatively large set of transitive dependencies.
Even though these are brought in as a weak dependency
(i.e., there is no hard code dependency on genproto),
it complicates dependency inspection since users need to vet
modules that they may not actually be using.

Avoid this weak dependency and instead rely on a dynamic check
to ensure that a sufficiently new version of genproto is used.
While it can't statically prevent old versions of genproto from being
linked into a binary, it guarantees that it will panic at init
with a helpful message on what's wrong.

Change-Id: I2b5a9759a5cd4c4c57aced54278502b1b6352ba7
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/262679
Trust: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/go.mod b/go.mod
index fcd5204..eb2af5a 100644
--- a/go.mod
+++ b/go.mod
@@ -5,5 +5,4 @@
 require (
 	github.com/golang/protobuf v1.4.1
 	github.com/google/go-cmp v0.5.0
-	google.golang.org/genproto v0.0.0-20200526211855-cb27e3aa2013
 )
diff --git a/go.sum b/go.sum
index 65a77ad..f2fbac2 100644
--- a/go.sum
+++ b/go.sum
@@ -1,13 +1,3 @@
-cloud.google.com/go v0.26.0/go.mod h1:aQUYkXzVsufM+DwF1aE+0xfcU+56JwCaLick0ClmMTw=
-github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03qcyfWMU=
-github.com/census-instrumentation/opencensus-proto v0.2.1/go.mod h1:f6KPmirojxKA12rnyqOA5BBL4O983OfeGPqjHWSTneU=
-github.com/client9/misspell v0.3.4/go.mod h1:qj6jICC3Q7zFZvVWo7KLAzC3yx5G7kyvSDkc90ppPyw=
-github.com/envoyproxy/go-control-plane v0.9.1-0.20191026205805-5f8ba28d4473/go.mod h1:YTl/9mNaCwkRvm6d1a2C3ymFceY/DCBVvsKhRF0iEA4=
-github.com/envoyproxy/protoc-gen-validate v0.1.0/go.mod h1:iSmxcyjqTsJpI2R4NaDN7+kN2VEUnK/pcBlmesArF7c=
-github.com/golang/glog v0.0.0-20160126235308-23def4e6c14b/go.mod h1:SBH7ygxi8pfUlaOkMMuAQtPIUF8ecWP5IEl/CR7VP2Q=
-github.com/golang/mock v1.1.1/go.mod h1:oTYuIxOrZwtPieC+H1uAHpcLFnEyAGVDL/k47Jfbm0A=
-github.com/golang/protobuf v1.2.0/go.mod h1:6lQm79b+lXiMfvg/cZm0SGofjICqVBUtrP5yJMmIC1U=
-github.com/golang/protobuf v1.3.2/go.mod h1:6lQm79b+lXiMfvg/cZm0SGofjICqVBUtrP5yJMmIC1U=
 github.com/golang/protobuf v1.4.0-rc.1/go.mod h1:ceaxUfeHdC40wWswd/P6IGgMaK3YpKi5j83Wpe3EHw8=
 github.com/golang/protobuf v1.4.0-rc.1.0.20200221234624-67d41d38c208/go.mod h1:xKAWHe0F5eneWXFV3EuXVDTCmh+JuBKY0li0aMyXATA=
 github.com/golang/protobuf v1.4.0-rc.2/go.mod h1:LlEzMj4AhA7rCAGe4KMBDvJI+AwstrUpVNzEA03Pprs=
@@ -15,50 +5,16 @@
 github.com/golang/protobuf v1.4.0/go.mod h1:jodUvKwWbYaEsadDk5Fwe5c77LiNKVO9IDvqG2KuDX0=
 github.com/golang/protobuf v1.4.1 h1:ZFgWrT+bLgsYPirOnRfKLYJLvssAegOj/hgyMFdJZe0=
 github.com/golang/protobuf v1.4.1/go.mod h1:U8fpvMrcmy5pZrNK1lt4xCsGvpyWQ/VVv6QDs8UjoX8=
-github.com/google/go-cmp v0.2.0/go.mod h1:oXzfMopK8JAjlY9xF4vHSVASa0yLyX7SntLO5aqRK0M=
 github.com/google/go-cmp v0.3.0/go.mod h1:8QqcDgzrUqlUb/G2PQTWiueGozuR1884gddMywk6iLU=
 github.com/google/go-cmp v0.3.1/go.mod h1:8QqcDgzrUqlUb/G2PQTWiueGozuR1884gddMywk6iLU=
 github.com/google/go-cmp v0.4.0/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
 github.com/google/go-cmp v0.5.0 h1:/QaMHBdZ26BB3SSst0Iwl10Epc+xhTquomWX0oZEB6w=
 github.com/google/go-cmp v0.5.0/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
-github.com/prometheus/client_model v0.0.0-20190812154241-14fe0d1b01d4/go.mod h1:xMI15A0UPsDsEKsMN9yxemIoYk6Tm2C1GtYGdfGttqA=
-golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
-golang.org/x/exp v0.0.0-20190121172915-509febef88a4/go.mod h1:CJ0aWSM057203Lf6IL+f9T1iT9GByDxfZKAQTCR3kQA=
-golang.org/x/lint v0.0.0-20181026193005-c67002cb31c3/go.mod h1:UVdnD1Gm6xHRNCYTkRU2/jEulfH38KcIWyp/GAMgvoE=
-golang.org/x/lint v0.0.0-20190227174305-5b3e6a55c961/go.mod h1:wehouNa3lNwaWXcvxsM5YxQ5yQlVC4a0KAMCusXpPoU=
-golang.org/x/lint v0.0.0-20190313153728-d0100b6bd8b3/go.mod h1:6SW0HCj/g11FgYtHlgUYUwCkIfeOF89ocIRzGO/8vkc=
-golang.org/x/net v0.0.0-20180724234803-3673e40ba225/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
-golang.org/x/net v0.0.0-20180826012351-8a410e7b638d/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
-golang.org/x/net v0.0.0-20190213061140-3a22650c66bd/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
-golang.org/x/net v0.0.0-20190311183353-d8887717615a/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
-golang.org/x/oauth2 v0.0.0-20180821212333-d2e6202438be/go.mod h1:N/0e6XlmueqKjAGxoOufVs8QHGRruUQn6yWY3a++T0U=
-golang.org/x/sync v0.0.0-20180314180146-1d60e4601c6f/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
-golang.org/x/sync v0.0.0-20181108010431-42b317875d0f/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
-golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
-golang.org/x/sys v0.0.0-20180830151530-49385e6e1522/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
-golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
-golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
-golang.org/x/tools v0.0.0-20190114222345-bf090417da8b/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
-golang.org/x/tools v0.0.0-20190226205152-f727befe758c/go.mod h1:9Yl7xja0Znq3iFh3HoIrodX9oNMXvdceNzlUR8zjMvY=
-golang.org/x/tools v0.0.0-20190311212946-11955173bddd/go.mod h1:LCzVGOaR6xXOjkQ3onu1FJEFr0SW1gC7cKk1uF8kGRs=
-golang.org/x/tools v0.0.0-20190524140312-2c0ae7006135/go.mod h1:RgjU9mgBXZiqYHBnxXauZ1Gv1EHHAz9KjViQ78xBX0Q=
 golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543 h1:E7g+9GITq07hpfrRu66IVDexMakfv52eLZ2CXBWiKr4=
 golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
-google.golang.org/appengine v1.1.0/go.mod h1:EbEs0AVv82hx2wNQdGPgUI5lhzA/G0D9YwlJXL52JkM=
-google.golang.org/appengine v1.4.0/go.mod h1:xpcJRLb0r/rnEns0DIKYYv+WjYCduHsrkT7/EB5XEv4=
-google.golang.org/genproto v0.0.0-20180817151627-c66870c02cf8/go.mod h1:JiN7NxoALGmiZfu7CAH4rXhgtRTLTxftemlI0sWmxmc=
-google.golang.org/genproto v0.0.0-20190819201941-24fa4b261c55/go.mod h1:DMBHOl98Agz4BDEuKkezgsaosCRResVns1a3J2ZsMNc=
-google.golang.org/genproto v0.0.0-20200526211855-cb27e3aa2013 h1:+kGHl1aib/qcwaRi1CbqBZ1rk19r85MNUf8HaBghugY=
-google.golang.org/genproto v0.0.0-20200526211855-cb27e3aa2013/go.mod h1:NbSheEEYHJ7i3ixzK3sjbqSGDJWnxyFXZblF3eUsNvo=
-google.golang.org/grpc v1.19.0/go.mod h1:mqu4LbDTu4XGKhr4mRzUsmM4RtVoemTSY81AxZiDr8c=
-google.golang.org/grpc v1.23.0/go.mod h1:Y5yQAOtifL1yxbo5wqy6BxZv8vAUGQwXBOALyacEbxg=
-google.golang.org/grpc v1.27.0/go.mod h1:qbnxyOmOxrQa7FizSgH+ReBfzJrCY1pSN7KXBS8abTk=
 google.golang.org/protobuf v0.0.0-20200109180630-ec00e32a8dfd/go.mod h1:DFci5gLYBciE7Vtevhsrf46CRTquxDuWsQurQQe4oz8=
 google.golang.org/protobuf v0.0.0-20200221191635-4d8936d0db64/go.mod h1:kwYJMbMJ01Woi6D6+Kah6886xMZcty6N08ah7+eCXa0=
 google.golang.org/protobuf v0.0.0-20200228230310-ab0ca4ff8a60/go.mod h1:cfTl7dwQJ+fmap5saPgwCLgHXTUD7jkjRqWcaiX5VyM=
 google.golang.org/protobuf v1.20.1-0.20200309200217-e05f789c0967/go.mod h1:A+miEFZTKqfCUM6K7xSMQL9OKL/b6hQv+e19PK+JZNE=
 google.golang.org/protobuf v1.21.0/go.mod h1:47Nbq4nVaFHyn7ilMalzfO3qCViNmqZ2kzikPIcrTAo=
 google.golang.org/protobuf v1.22.0/go.mod h1:EGpADcykh3NcUnDUJcl1+ZksZNG86OlYog2l/sGQquU=
-google.golang.org/protobuf v1.23.1-0.20200526195155-81db48ad09cc/go.mod h1:EGpADcykh3NcUnDUJcl1+ZksZNG86OlYog2l/sGQquU=
-honnef.co/go/tools v0.0.0-20190102054323-c2f93a96b099/go.mod h1:rf3lG4BRIbNafJWhAfAdb/ePZxsR/4RtNHQocxwk9r4=
-honnef.co/go/tools v0.0.0-20190523083050-ea95bdfd59fc/go.mod h1:rf3lG4BRIbNafJWhAfAdb/ePZxsR/4RtNHQocxwk9r4=
diff --git a/internal/weakdeps/weakdeps.go b/internal/weakdeps/weakdeps.go
index 47c12cf..59b3475 100644
--- a/internal/weakdeps/weakdeps.go
+++ b/internal/weakdeps/weakdeps.go
@@ -11,12 +11,4 @@
 	// uses a version that wraps this module so that there is a
 	// unified view on what protobuf types are globally registered.
 	_ "github.com/golang/protobuf/proto"
-
-	// Ensure that any program using "google.golang.org/genproto"
-	// uses a version that forwards their generated well-known types
-	// to reference the ones declared in this module.
-	_ "google.golang.org/genproto/protobuf/api"
-	_ "google.golang.org/genproto/protobuf/field_mask"
-	_ "google.golang.org/genproto/protobuf/ptype"
-	_ "google.golang.org/genproto/protobuf/source_context"
 )
diff --git a/reflect/protoregistry/registry.go b/reflect/protoregistry/registry.go
index be26105..fbd0fec 100644
--- a/reflect/protoregistry/registry.go
+++ b/reflect/protoregistry/registry.go
@@ -98,38 +98,7 @@
 	}
 	path := file.Path()
 	if prev := r.filesByPath[path]; prev != nil {
-		// TODO: Remove this after some soak-in period after moving these types.
-		var prevPath string
-		const prevModule = "google.golang.org/genproto"
-		const prevVersion = "cb27e3aa (May 26th, 2020)"
-		switch path {
-		case "google/protobuf/field_mask.proto":
-			prevPath = prevModule + "/protobuf/field_mask"
-		case "google/protobuf/api.proto":
-			prevPath = prevModule + "/protobuf/api"
-		case "google/protobuf/type.proto":
-			prevPath = prevModule + "/protobuf/ptype"
-		case "google/protobuf/source_context.proto":
-			prevPath = prevModule + "/protobuf/source_context"
-		}
-		if r == GlobalFiles && prevPath != "" {
-			pkgName := strings.TrimSuffix(strings.TrimPrefix(path, "google/protobuf/"), ".proto")
-			pkgName = strings.Replace(pkgName, "_", "", -1) + "pb"
-			currPath := "google.golang.org/protobuf/types/known/" + pkgName
-			panic(fmt.Sprintf(""+
-				"duplicate registration of %q\n"+
-				"\n"+
-				"The generated definition for this file has moved:\n"+
-				"\tfrom: %q\n"+
-				"\tto:   %q\n"+
-				"A dependency on the %q module must\n"+
-				"be at version %v or higher.\n"+
-				"\n"+
-				"Upgrade the dependency by running:\n"+
-				"\tgo get -u %v\n",
-				path, prevPath, currPath, prevModule, prevVersion, prevPath))
-		}
-
+		r.checkGenProtoConflict(path)
 		err := errors.New("file %q is already registered", file.Path())
 		err = amendErrorWithCaller(err, prev, file)
 		if r == GlobalFiles && ignoreConflict(file, err) {
@@ -180,6 +149,47 @@
 	return nil
 }
 
+// Several well-known types were hosted in the google.golang.org/genproto module
+// but were later moved to this module. To avoid a weak dependency on the
+// genproto module (and its relatively large set of transitive dependencies),
+// we rely on a registration conflict to determine whether the genproto version
+// is too old (i.e., does not contain aliases to the new type declarations).
+func (r *Files) checkGenProtoConflict(path string) {
+	if r != GlobalFiles {
+		return
+	}
+	var prevPath string
+	const prevModule = "google.golang.org/genproto"
+	const prevVersion = "cb27e3aa (May 26th, 2020)"
+	switch path {
+	case "google/protobuf/field_mask.proto":
+		prevPath = prevModule + "/protobuf/field_mask"
+	case "google/protobuf/api.proto":
+		prevPath = prevModule + "/protobuf/api"
+	case "google/protobuf/type.proto":
+		prevPath = prevModule + "/protobuf/ptype"
+	case "google/protobuf/source_context.proto":
+		prevPath = prevModule + "/protobuf/source_context"
+	default:
+		return
+	}
+	pkgName := strings.TrimSuffix(strings.TrimPrefix(path, "google/protobuf/"), ".proto")
+	pkgName = strings.Replace(pkgName, "_", "", -1) + "pb" // e.g., "field_mask" => "fieldmaskpb"
+	currPath := "google.golang.org/protobuf/types/known/" + pkgName
+	panic(fmt.Sprintf(""+
+		"duplicate registration of %q\n"+
+		"\n"+
+		"The generated definition for this file has moved:\n"+
+		"\tfrom: %q\n"+
+		"\tto:   %q\n"+
+		"A dependency on the %q module must\n"+
+		"be at version %v or higher.\n"+
+		"\n"+
+		"Upgrade the dependency by running:\n"+
+		"\tgo get -u %v\n",
+		path, prevPath, currPath, prevModule, prevVersion, prevPath))
+}
+
 // FindDescriptorByName looks up a descriptor by the full name.
 //
 // This returns (nil, NotFound) if not found.