cmd/go: canonicalize implied toolchain versions

src/cmd/go/internal/gover/toolchain.go

@@ -45,6 +45,15 @@ func FromToolchain(name string) string {
 	return v
 }
 
+// ToolchainForGoVersion returns the implied toolchain name for the Go version v.
+func ToolchainForGoVersion(v string) string {
+	name := "go" + v
+	if IsLang(v) && Compare(v, GoStrictVersion) >= 0 {
+		name += ".0"
+	}
+	return name
+}
+
 func maybeToolchainVersion(name string) string {
 	if IsValid(name) {
 		return name

src/cmd/go/internal/gover/toolchain_test.go

@@ -17,3 +17,16 @@ var fromToolchainTests = []testCase1[string, string]{
 	{"gccgo-go1.23rc4", ""},
 	{"gccgo-go1.23rc4-bigdwarf", ""},
 }
+
+func TestToolchainForGoVersion(t *testing.T) {
+	test1(t, toolchainForGoVersionTests, "ToolchainForGoVersion", ToolchainForGoVersion)
+}
+
+var toolchainForGoVersionTests = []testCase1[string, string]{
+	{"1.20", "go1.20"},
+	{"1.21", "go1.21.0"},
+	{"1.22", "go1.22.0"},
+	{"1.23", "go1.23.0"},
+	{"1.23.4", "go1.23.4"},
+	{"1.24rc1", "go1.24rc1"},
+}

src/cmd/go/internal/modload/init.go

@@ -918,8 +918,9 @@ func UpdateWorkGoVersion(wf *modfile.WorkFile, goVers string) (changed bool) {
 	// This behavior seemed to cause user confusion without much
 	// benefit so it was removed. See #65847.
 	toolchain := wf.Toolchain.Name
+	impliedToolchain := gover.ToolchainForGoVersion(goVers)
 	toolVers := gover.FromToolchain(toolchain)
-	if toolchain == "go"+goVers || gover.Compare(toolVers, goVers) < 0 || gover.Compare(toolVers, gover.GoStrictVersion) < 0 {
+	if toolchain == "go"+goVers || toolchain == impliedToolchain || gover.Compare(toolVers, goVers) < 0 || gover.Compare(toolVers, gover.GoStrictVersion) < 0 {
 		wf.DropToolchainStmt()
 	}
 
@@ -2012,11 +2013,12 @@ func UpdateGoModFromReqs(ld *Loader, ctx context.Context, opts WriteOpts) (befor
 		forceGoStmt(modFile, mainModule, gover.GoModToolVersion)
 	}
 
+	impliedToolchain := gover.ToolchainForGoVersion(goVersion)
 	if toolchain == "" {
-		toolchain = "go" + goVersion
+		toolchain = impliedToolchain
 	}
 	toolVers := gover.FromToolchain(toolchain)
-	if opts.DropToolchain || toolchain == "go"+goVersion || (gover.Compare(toolVers, gover.GoStrictVersion) < 0 && !opts.ExplicitToolchain) {
+	if opts.DropToolchain || toolchain == "go"+goVersion || toolchain == impliedToolchain || (gover.Compare(toolVers, gover.GoStrictVersion) < 0 && !opts.ExplicitToolchain) {
 		// go get toolchain@none or toolchain matches go line or isn't valid; drop it.
 		// TODO(#57001): 'go get' should reject explicit toolchains below GoStrictVersion.
 		modFile.DropToolchainStmt()

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

@@ -682,7 +682,7 @@ func rawGoModSummary(ld *Loader, m module.Version) (*modFileSummary, error) {
 			// Declare that go 1.21.3 requires toolchain 1.21.3,
 			// so that go get knows that downgrading toolchain implies downgrading go
 			// and similarly upgrading go requires upgrading the toolchain.
-			return &modFileSummary{module: m, require: []module.Version{{Path: "toolchain", Version: "go" + m.Version}}}, nil
+			return &modFileSummary{module: m, require: []module.Version{{Path: "toolchain", Version: gover.ToolchainForGoVersion(m.Version)}}}, nil
 		}
 		return &modFileSummary{module: m}, nil
 	}

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

@@ -214,15 +214,12 @@ func Select() {
 				}
 			}
 			if gover.Compare(goVers, minVers) > 0 {
-				gotoolchain = "go" + goVers
-				minVers = goVers
 				// Starting with Go 1.21, the first released version has a .0 patch version suffix.
 				// Don't try to download a language version (sans patch component), such as go1.22.
 				// Instead, use the first toolchain of that language version, such as 1.22.0.
 				// See golang.org/issue/62278.
-				if gover.IsLang(goVers) && gover.Compare(goVers, "1.21") >= 0 {
-					gotoolchain += ".0"
-				}
+				gotoolchain = gover.ToolchainForGoVersion(goVers)
+				minVers = goVers
 				gover.Startup.AutoGoVersion = goVers
 				gover.Startup.AutoToolchain = "" // in case we are overriding it for being too old
 				if toolchainTrace {

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

@@ -105,6 +105,17 @@ grep 'toolchain go1.23.9' go.mod
 
 # Bug fixes.
 
+# go get go@version should not write a language version as the toolchain when
+# preserving the original go line's implied toolchain.
+cp go.mod.oldlang go.mod
+go get go@1.22.9
+stderr '^go: downgraded go 1.23 => 1.22.9$'
+stderr '^go: upgraded toolchain go1.22.9 => go1.23.0$'
+grep '^go 1.22.9$' go.mod
+grep '^toolchain go1.23.0$' go.mod
+! grep '^toolchain go1.23$' go.mod
+cp go.mod.latest go.mod
+
 # go get go@garbage should fail but not crash
 ! go get go@garbage
 ! stderr panic
@@ -134,3 +145,17 @@ grep 'go 1.21$' go.mod
 module m
 
 go 1.21
+
+-- go.mod.oldlang --
+module m
+
+go 1.23
+
+toolchain go1.22.9
+
+-- go.mod.latest --
+module m
+
+go 1.21
+
+toolchain go1.23.9