cmd/go: check go version when parsing go.mod fails

Fixes #70979

Change-Id: I6597fe178eed34702eea6cba4eec5174c9203458
Reviewed-on: https://go-review.googlesource.com/c/go/+/639115
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/cmd/go/internal/modload/modfile.go

@@ -44,6 +44,17 @@ func ReadModFile(gomod string, fix modfile.VersionFixer) (data []byte, f *modfil
 
 	f, err = modfile.Parse(gomod, data, fix)
 	if err != nil {
+		f, laxErr := modfile.ParseLax(gomod, data, fix)
+		if laxErr == nil {
+			if f.Go != nil && gover.Compare(f.Go.Version, gover.Local()) > 0 {
+				toolchain := ""
+				if f.Toolchain != nil {
+					toolchain = f.Toolchain.Name
+				}
+				return nil, nil, &gover.TooNewError{What: base.ShortPath(gomod), GoVersion: f.Go.Version, Toolchain: toolchain}
+			}
+		}
+
 		// Errors returned by modfile.Parse begin with file:line.
 		return nil, nil, fmt.Errorf("errors parsing %s:\n%w", base.ShortPath(gomod), shortPathErrorList(err))
 	}

src/cmd/go/testdata/script/mod_unknown_block.txt

@@ -0,0 +1,11 @@
+env GOTOOLCHAIN=local
+! go list .
+stderr 'go: go.mod requires go >= 1.999'
+
+
+-- go.mod --
+module example.com
+
+go 1.999
+
+anewblock foo