cmd/go: allow 'go get' to downgrade to replacement-only versions

This fixes a case missed in CL 258220.

For #36460
Updates #26241
Updates #37438

Change-Id: I5e8c2ee1e08e41cc2eb34e54c617cb5e4bf69c5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/266018
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

3 files changed, 152 insertions, 3 deletions

diff --git a/src/cmd/go/internal/modload/mvs.go b/src/cmd/go/internal/modload/mvs.go
index 76a1d8a12a..79ef2475b6 100644
--- a/src/cmd/go/internal/modload/mvs.go
+++ b/src/cmd/go/internal/modload/mvs.go
@@ -74,7 +74,11 @@ func versions(ctx context.Context, path string, allowed AllowedFunc) ([]string,
 	// so there's no need for us to add extra caching here.
 	var versions []string
 	err := modfetch.TryProxies(func(proxy string) error {
-		allVersions, err := modfetch.Lookup(proxy, path).Versions("")
+		repo, err := lookupRepo(proxy, path)
+		if err != nil {
+			return err
+		}
+		allVersions, err := repo.Versions("")
 		if err != nil {
 			return err
 		}
diff --git a/src/cmd/go/testdata/script/mod_get_replaced.txt b/src/cmd/go/testdata/script/mod_get_replaced.txt
index ea4c603795..f838605900 100644
--- a/src/cmd/go/testdata/script/mod_get_replaced.txt
+++ b/src/cmd/go/testdata/script/mod_get_replaced.txt
@@ -48,7 +48,7 @@ go list -m example.com/x
 stdout '^example.com/x v0.1.0 '
 
 
-# Replacements should not be listed as known versions, but 'go get' should sort
+# Replacements should also be listed as known versions, and 'go get' should sort
 # them in with ordinary versions.
 
 env GOPROXY=$oldGOPROXY
@@ -64,7 +64,7 @@ stdout '^rsc.io/quote v1.3.0'
 go mod edit -replace rsc.io/quote@v1.3.1=rsc.io/quote@v1.4.0
 
 go list -versions -m rsc.io/quote
-stdout 'v1.3.0 v1.4.0'
+stdout 'v1.3.0 v1.3.1 v1.4.0'
 
 go get -d rsc.io/quote@v1.3
 go list -m rsc.io/quote
diff --git a/src/cmd/go/testdata/script/mod_lazy_downgrade.txt b/src/cmd/go/testdata/script/mod_lazy_downgrade.txt
new file mode 100644
index 0000000000..1e84820f81
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_lazy_downgrade.txt
@@ -0,0 +1,145 @@
+# This test illustrates the interaction between lazy loading and downgrading in
+# 'go get.
+
+# The package import graph used in this test looks like:
+#
+# lazy ---- a
+#           |
+#           a_test ---- b
+#                       b_test ---- c
+#
+# The module dependency graph initially looks like:
+#
+# lazy ---- a.1 ---- b.1 ---- c.1
+#      \                     /
+#        b.3 ---- c.2    b.2
+#
+# (Note that lazy loading will prune out the dependency from b.1 on c.1.)
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod.orig go.mod
+
+go list -m all
+stdout '^example.com/a v0.1.0 '
+stdout '^example.com/b v0.3.0 '
+stdout '^example.com/c v0.2.0 '
+
+# Downgrading c should also downgrade the b that requires it.
+
+go get -d example.com/c@v0.1.0
+go list -m all
+stdout '^example.com/a v0.1.0 '
+stdout '^example.com/b v0.2.0 '
+stdout '^example.com/c v0.1.0 '
+
+# Removing c entirely should also remove the a and b that require it.
+
+go get -d example.com/c@none
+go list -m all
+! stdout '^example.com/a '
+! stdout '^example.com/b '
+! stdout '^example.com/c '
+
+
+# With lazy loading, downgrading c should work the same way, but dependencies
+# outside of the deepening scan should not affect the downgrade.
+
+cp go.mod.orig go.mod
+go mod edit -go=1.16
+
+go list -m all
+stdout '^example.com/a v0.1.0 '
+stdout '^example.com/b v0.3.0 '
+stdout '^example.com/c v0.2.0 '
+
+go get -d example.com/c@v0.1.0
+go list -m all
+stdout '^example.com/a v0.1.0 '
+stdout '^example.com/b v0.2.0 '
+stdout '^example.com/c v0.1.0 '
+
+go get -d example.com/c@none
+go list -m all
+! stdout '^example.com/a '  # TODO(#36460): example.com/a v0.1.0
+! stdout '^example.com/b '  # TODO(#36460): example.com/b v0.1.0
+! stdout '^example.com/c '
+
+-- go.mod --
+module example.com/lazy
+
+go 1.15
+
+require (
+	example.com/a v0.1.0
+	example.com/b v0.3.0 // indirect
+)
+
+replace (
+	example.com/a v0.1.0 => ./a
+	example.com/b v0.1.0 => ./b1
+	example.com/b v0.2.0 => ./b2
+	example.com/b v0.3.0 => ./b3
+	example.com/c v0.1.0 => ./c
+	example.com/c v0.2.0 => ./c
+)
+-- lazy.go --
+package lazy
+
+import _ "example.com/a"
+
+-- a/go.mod --
+module example.com/a
+
+go 1.15
+
+require example.com/b v0.1.0
+-- a/a.go --
+package a
+-- a/a_test.go --
+package a_test
+
+import _ "example.com/b"
+
+-- b1/go.mod --
+module example.com/b
+
+go 1.15
+
+require example.com/c v0.1.0
+-- b1/b.go --
+package b
+-- b1/b_test.go --
+package b_test
+import _ "example.com/c"
+
+-- b2/go.mod --
+module example.com/b
+
+go 1.15
+
+require example.com/c v0.1.0
+-- b2/b.go --
+package b
+-- b2/b_test.go --
+package b_test
+import _ "example.com/c"
+
+-- b3/go.mod --
+module example.com/b
+
+go 1.15
+
+require example.com/c v0.2.0
+-- b3/b.go --
+package b
+-- b3/b_test.go --
+package b_test
+import _ "example.com/c"
+
+-- c/go.mod --
+module example.com/c
+
+go 1.15
+-- c/c.go --
+package c