cmd/go: allow suffixed toolchains to satisfy toolchain lines for the same base version

Fixes #63357.

Change-Id: I8380cf0d3965d6aef84a91a515d3e0e8aae9344b
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/535355
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: Bryan C. Mills

src/cmd/go/internal/toolchain/select.go

@@ -167,12 +167,14 @@ func Select() {
 			gover.Startup.AutoToolchain = toolchain
 		} else {
 			if toolchain != "" {
-				// Accept toolchain only if it is >= our min.
+				// Accept toolchain only if it is > our min.
+				// (If it is equal, then min satisfies it anyway: that can matter if min
+				// has a suffix like "go1.21.1-foo" and toolchain is "go1.21.1".)
 				toolVers := gover.FromToolchain(toolchain)
 				if toolVers == "" || (!strings.HasPrefix(toolchain, "go") && !strings.Contains(toolchain, "-go")) {
 					base.Fatalf("invalid toolchain %q in %s", toolchain, base.ShortPath(file))
 				}
-				if gover.Compare(toolVers, minVers) >= 0 {
+				if gover.Compare(toolVers, minVers) > 0 {
 					gotoolchain = toolchain
 					minVers = toolVers
 					gover.Startup.AutoToolchain = toolchain

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

@@ -163,12 +163,25 @@ go mod edit -go=1.499 -toolchain=none
 go version
 stdout go1.500-bigcorp
 
+go mod edit -go=1.499 -toolchain=go1.499
+go version
+stdout go1.500-bigcorp
+
 go mod edit -go=1.500 -toolchain=none
 go version
 stdout go1.500-bigcorp
 
+go mod edit -go=1.500 -toolchain=go1.500
+go version
+stdout go1.500-bigcorp
+
 go mod edit -go=1.501 -toolchain=none
 go version
+stdout go1.501
+
+	# If toolchain > go, we must upgrade to the indicated toolchain (not just the go version).
+go mod edit -go=1.499 -toolchain=go1.501
+go version
 stdout go1.501
 
 env TESTGO_VERSION='go1.500 (bigcorp)'