[release-branch.go1.15] cmd/go: error out of 'go mod tidy' if the go version is newer than supported

This backports the test from CL 319669, but — because of extensive
changes to the module loader during the Go 1.16 and 1.17 cycles — the
implementation is entirely different. (This implementation is based on
the addGoStmt function already present in init.go.)

Fixes #46143
Updates #46142

Change-Id: Ib7a0a159e53cbe476be6aa9a050add10cc750dec
Reviewed-on: https://go-review.googlesource.com/c/go/+/319710
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

1 files changed, 31 insertions, 0 deletions

diff --git a/src/cmd/go/testdata/script/mod_tidy_too_new.txt b/src/cmd/go/testdata/script/mod_tidy_too_new.txt
new file mode 100644
index 0000000000..8a73c28cbb
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_too_new.txt
@@ -0,0 +1,31 @@
+# https://golang.org/issue/46142: 'go mod tidy' should error out if the version
+# in the go.mod file is newer than the most recent supported version.
+
+cp go.mod go.mod.orig
+
+
+# If the go.mod file specifies an unsupported Go version, 'go mod tidy' should
+# refuse to edit it: we don't know what a tidy go.mod file for that version
+# would look like.
+
+! go mod tidy
+stderr 'go mod tidy: go.mod file indicates go 2000.0, but maximum supported version is '$goversion'$'
+cmp go.mod go.mod.orig
+
+
+-- go.mod --
+module example.net/from/the/future
+
+go 2000.0
+
+replace example.net/m v0.0.0 => ./m
+-- x.go --
+package x
+
+import "example.net/m"
+-- m/go.mod --
+module example.net/m
+
+go 1.17
+-- m/m.go --
+package m