diff --git a/doc/go1.13.html b/doc/go1.13.html
index 4240d4b..69bb1b0 100644
--- a/doc/go1.13.html
+++ b/doc/go1.13.html
@@ -302,8 +302,10 @@
   between <a href="/cmd/go#hdr-Pseudo_versions">pseudo-versions</a> and
   version-control metadata. Specifically:
   <ul>
-    <li>The version prefix must be derived from a tag on the named revision or
-    one of its ancestors, or be of the form <code>vX.0.0</code>.</li>
+    <li>The version prefix must be of the form <code>vX.0.0</code>, or derived
+    from a tag on an ancestor of the named revision, or derived from a tag that
+    includes <a href="https://semver.org/#spec-item-10">build metadata</a> on
+    the named revision itself.</li>
 
     <li>The date string must match the UTC timestamp of the revision.</li>
 
diff --git a/src/cmd/go/internal/modfetch/coderepo.go b/src/cmd/go/internal/modfetch/coderepo.go
index 491fe11..548c684 100644
--- a/src/cmd/go/internal/modfetch/coderepo.go
+++ b/src/cmd/go/internal/modfetch/coderepo.go
@@ -479,6 +479,11 @@
 		return fmt.Errorf("does not match version-control timestamp (%s)", info.Time.UTC().Format(time.RFC3339))
 	}
 
+	tagPrefix := ""
+	if r.codeDir != "" {
+		tagPrefix = r.codeDir + "/"
+	}
+
 	// A pseudo-version should have a precedence just above its parent revisions,
 	// and no higher. Otherwise, it would be possible for library authors to "pin"
 	// dependency versions (and bypass the usual minimum version selection) by
@@ -504,11 +509,26 @@
 			return fmt.Errorf("major version without preceding tag must be v0, not v1")
 		}
 		return nil
-	}
-
-	tagPrefix := ""
-	if r.codeDir != "" {
-		tagPrefix = r.codeDir + "/"
+	} else {
+		for _, tag := range info.Tags {
+			versionOnly := strings.TrimPrefix(tag, tagPrefix)
+			if versionOnly == base {
+				// The base version is canonical, so if the version from the tag is
+				// literally equal (not just equivalent), then the tag is canonical too.
+				//
+				// We allow pseudo-versions to be derived from non-canonical tags on the
+				// same commit, so that tags like "v1.1.0+some-metadata" resolve as
+				// close as possible to the canonical version ("v1.1.0") while still
+				// enforcing a total ordering ("v1.1.1-0.[…]" with a unique suffix).
+				//
+				// However, canonical tags already have a total ordering, so there is no
+				// reason not to use the canonical tag directly, and we know that the
+				// canonical tag must already exist because the pseudo-version is
+				// derived from it. In that case, referring to the revision by a
+				// pseudo-version derived from its own canonical tag is just confusing.
+				return fmt.Errorf("tag (%s) found on revision %s is already canonical, so should not be replaced with a pseudo-version derived from that tag", tag, rev)
+			}
+		}
 	}
 
 	tags, err := r.code.Tags(tagPrefix + base)
diff --git a/src/cmd/go/internal/modfetch/coderepo_test.go b/src/cmd/go/internal/modfetch/coderepo_test.go
index b5c9be5..1f2b95b 100644
--- a/src/cmd/go/internal/modfetch/coderepo_test.go
+++ b/src/cmd/go/internal/modfetch/coderepo_test.go
@@ -86,6 +86,26 @@
 	{
 		vcs:     "git",
 		path:    "github.com/rsc/vgotest1",
+		rev:     "v0.0.0-20180219231006-80d85c5d4d17",
+		version: "v0.0.0-20180219231006-80d85c5d4d17",
+		name:    "80d85c5d4d17598a0e9055e7c175a32b415d6128",
+		short:   "80d85c5d4d17",
+		time:    time.Date(2018, 2, 19, 23, 10, 6, 0, time.UTC),
+		zip: []string{
+			"LICENSE",
+			"README.md",
+			"pkg/p.go",
+		},
+	},
+	{
+		vcs:  "git",
+		path: "github.com/rsc/vgotest1",
+		rev:  "v0.0.1-0.20180219231006-80d85c5d4d17",
+		err:  `github.com/rsc/vgotest1@v0.0.1-0.20180219231006-80d85c5d4d17: invalid pseudo-version: tag (v0.0.0) found on revision 80d85c5d4d17 is already canonical, so should not be replaced with a pseudo-version derived from that tag`,
+	},
+	{
+		vcs:     "git",
+		path:    "github.com/rsc/vgotest1",
 		rev:     "v1.0.0",
 		version: "v1.0.0",
 		name:    "80d85c5d4d17598a0e9055e7c175a32b415d6128",
@@ -515,7 +535,7 @@
 }
 
 var hgmap = map[string]string{
-	"github.com/rsc/vgotest1/":                 "vcs-test.golang.org/hg/vgotest1.hg/",
+	"github.com/rsc/vgotest1":                  "vcs-test.golang.org/hg/vgotest1.hg",
 	"f18795870fb14388a21ef3ebc1d75911c8694f31": "a9ad6d1d14eb544f459f446210c7eb3b009807c6",
 	"ea65f87c8f52c15ea68f3bdd9925ef17e20d91e9": "f1fc0f22021b638d073d31c752847e7bf385def7",
 	"b769f2de407a4db81af9c5de0a06016d60d2ea09": "92c7eb888b4fac17f1c6bd2e1060a1b881a3b832",
diff --git a/src/cmd/go/testdata/script/mod_invalid_version.txt b/src/cmd/go/testdata/script/mod_invalid_version.txt
index 2be0d01..a587b44 100644
--- a/src/cmd/go/testdata/script/mod_invalid_version.txt
+++ b/src/cmd/go/testdata/script/mod_invalid_version.txt
@@ -144,6 +144,16 @@
 ! go list -m golang.org/x/text
 stderr 'golang.org/x/text@v0.2.1-0.20170915032832-14c0d48ead0c: invalid pseudo-version: revision 14c0d48ead0c is not a descendent of preceding tag \(v0.2.0\)'
 
+# A pseudo-version derived from a canonical tag on the same revision is invalid.
+cp go.mod.orig go.mod
+go mod edit -require golang.org/x/text@v0.2.1-0.20171213102548-c4d099d611ac
+cd outside
+! go list -m golang.org/x/text
+stderr 'go: example.com@v0.0.0 requires\n\tgolang.org/x/text@v0.2.1-0.20171213102548-c4d099d611ac: invalid pseudo-version: tag \(v0.2.0\) found on revision c4d099d611ac is already canonical, so should not be replaced with a pseudo-version derived from that tag'
+cd ..
+! go list -m golang.org/x/text
+stderr 'golang.org/x/text@v0.2.1-0.20171213102548-c4d099d611ac: invalid pseudo-version: tag \(v0.2.0\) found on revision c4d099d611ac is already canonical, so should not be replaced with a pseudo-version derived from that tag'
+
 # A +incompatible suffix is not allowed on a version that is actually compatible.
 cp go.mod.orig go.mod
 go mod edit -require golang.org/x/text@v0.1.1-0.20170915032832-14c0d48ead0c+incompatible
@@ -165,15 +175,15 @@
 stdout 'github.com/pierrec/lz4 v1.0.2-0.20190131084431-473cd7ce01a1'
 cd ..
 
-# A +incompatible version for a module that has an explicit go.mod file is invalid.
+# A +incompatible pseudo-version for a module that has an explicit go.mod file is invalid.
 cp go.mod.orig go.mod
-go mod edit -require github.com/pierrec/lz4@v2.0.9-0.20190131084431-473cd7ce01a1+incompatible
+go mod edit -require github.com/pierrec/lz4@v2.0.9-0.20190209155647-9a39efadad3d+incompatible
 cd outside
 ! go list -m github.com/pierrec/lz4
-stderr 'go: example.com@v0.0.0 requires\n\tgithub.com/pierrec/lz4@v2.0.9-0.20190131084431-473cd7ce01a1\+incompatible: invalid version: \+incompatible suffix not allowed: module contains a go.mod file, so semantic import versioning is required'
+stderr 'go: example.com@v0.0.0 requires\n\tgithub.com/pierrec/lz4@v2.0.9-0.20190209155647-9a39efadad3d\+incompatible: invalid version: \+incompatible suffix not allowed: module contains a go.mod file, so semantic import versioning is required'
 cd ..
 ! go list -m github.com/pierrec/lz4
-stderr 'github.com/pierrec/lz4@v2.0.9-0.20190131084431-473cd7ce01a1\+incompatible: invalid version: \+incompatible suffix not allowed: module contains a go.mod file, so semantic import versioning is required'
+stderr 'github.com/pierrec/lz4@v2.0.9-0.20190209155647-9a39efadad3d\+incompatible: invalid version: \+incompatible suffix not allowed: module contains a go.mod file, so semantic import versioning is required'
 
 # A +incompatible pseudo-version is valid for a revision of the module
 # that lacks a go.mod file.