[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>

src/cmd/go/internal/modcmd/tidy.go

@@ -43,6 +43,7 @@ func runTidy(cmd *base.Command, args []string) {
 	}
 
 	modload.SilenceMissingStdImports = true
+	modload.CheckTidyVersion()
 	modload.LoadALL()
 	modload.TidyBuildList()
 	modTidyGoSum() // updates memory copy; WriteGoMod on next line flushes it out

src/cmd/go/internal/modload/load.go

@@ -24,7 +24,9 @@ import (
 	"sort"
 	"strings"
 
+	"golang.org/x/mod/modfile"
 	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
 )
 
 // buildList is the list of modules to use for building packages.
@@ -478,6 +480,31 @@ func SetBuildList(list []module.Version) {
 	buildList = append([]module.Version{}, list...)
 }
 
+// CheckTidyVersion reports an error to stderr if the Go version indicated by
+// the go.mod file is not supported by this version of the 'go' command.
+//
+// If allowError is false, such an error terminates the program.
+func CheckTidyVersion() {
+	InitMod()
+	mf := ModFile()
+	if mf.Go == nil || mf.Go.Version == "" {
+		return
+	}
+	goVersionV := "v" + mf.Go.Version
+
+	tags := build.Default.ReleaseTags
+	maxGo := tags[len(tags)-1]
+	if !strings.HasPrefix(maxGo, "go") || !modfile.GoVersionRE.MatchString(maxGo[2:]) {
+		base.Fatalf("go: unrecognized go version %q", maxGo)
+	}
+	max := maxGo[2:]
+
+	if semver.Compare(goVersionV, "v"+max) > 0 {
+		have := goVersionV[1:]
+		base.Fatalf("go mod tidy: go.mod file indicates go %s, but maximum supported version is %s\n", have, max)
+	}
+}
+
 // TidyBuildList trims the build list to the minimal requirements needed to
 // retain the same versions of all packages from the preceding Load* or
 // ImportPaths* call.

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