go/types: flip the default value of GODEBUG=gotypesalias=1

This CL changes the interpretation of the unset value
of gotypesalias to equal "1". The actual deletion of
all the transitional logic will happen in a follow-up.

Note that the compiler still interprets unset as "0".
More work appears to be required within the compiler
before it is safe to flip its default.

Change-Id: I854ab1fd856c7c361a757676b0670e2f23402816
Reviewed-on: https://go-review.googlesource.com/c/go/+/577715
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Commit-Queue: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/go/internal/gcimporter/ureader.go

@@ -659,9 +659,10 @@ func pkgScope(pkg *types.Package) *types.Scope {
 
 // newAliasTypeName returns a new TypeName, with a materialized *types.Alias if supported.
 func newAliasTypeName(pos token.Pos, pkg *types.Package, name string, rhs types.Type) *types.TypeName {
-	// When GODEBUG=gotypesalias=1, the Type() of the return value is a
+	// When GODEBUG=gotypesalias=1 or unset, the Type() of the return value is a
 	// *types.Alias. Copied from x/tools/internal/aliases.NewAlias.
-	if godebug.New("gotypesalias").Value() == "1" {
+	switch godebug.New("gotypesalias").Value() {
+	case "", "1":
 		tname := types.NewTypeName(pos, pkg, name, nil)
 		_ = types.NewAlias(tname, rhs) // form TypeName -> Alias cycle
 		return tname

src/go/types/api_test.go

@@ -2997,7 +2997,6 @@ func TestTooNew(t *testing.T) {
 
 // This is a regression test for #66704.
 func TestUnaliasTooSoonInCycle(t *testing.T) {
-	t.Setenv("GODEBUG", "gotypesalias=1")
 	const src = `package a
 
 var x T[B] // this appears to cause Unalias to be called on B while still Invalid

src/go/types/check.go

@@ -23,7 +23,9 @@ var noposn = atPos(nopos)
 // debugging/development support
 const debug = false // leave on during development
 
-// gotypesalias controls the use of Alias types
+// gotypesalias controls the use of Alias types.
+// As of Apr 12 2024 it is on by default.
+// It will be removed soon.
 var gotypesalias = godebug.New("gotypesalias")
 
 // exprInfo stores information about an untyped expression.
@@ -258,8 +260,14 @@ func NewChecker(conf *Config, fset *token.FileSet, pkg *Package, info *Info) *Ch
 	//
 	// (previously, pkg.goVersion was mutated here: go.dev/issue/61212)
 
+	enableAlias := false
+	switch gotypesalias.Value() {
+	case "", "1":
+		enableAlias = true
+	}
+
 	return &Checker{
-		enableAlias: gotypesalias.Value() == "1",
+		enableAlias: enableAlias,
 		conf:        conf,
 		ctxt:        conf.Context,
 		fset:        fset,

src/go/types/check_test.go

@@ -124,8 +124,6 @@ func parseFlags(src []byte, flags *flag.FlagSet) error {
 
 // testFiles type-checks the package consisting of the given files, and
 // compares the resulting errors with the ERROR annotations in the source.
-// Except for manual tests, each package is type-checked twice, once without
-// use of Alias types, and once with Alias types.
 //
 // The srcs slice contains the file content for the files named in the
 // filenames slice. The colDelta parameter specifies the tolerance for position
@@ -134,15 +132,6 @@ func parseFlags(src []byte, flags *flag.FlagSet) error {
 //
 // If provided, opts may be used to mutate the Config before type-checking.
 func testFiles(t *testing.T, filenames []string, srcs [][]byte, manual bool, opts ...func(*Config)) {
-	// Alias types are disabled by default
-	testFilesImpl(t, filenames, srcs, manual, opts...)
-	if !manual {
-		t.Setenv("GODEBUG", "gotypesalias=1")
-		testFilesImpl(t, filenames, srcs, manual, opts...)
-	}
-}
-
-func testFilesImpl(t *testing.T, filenames []string, srcs [][]byte, manual bool, opts ...func(*Config)) {
 	if len(filenames) == 0 {
 		t.Fatal("no source files")
 	}

src/go/types/decl.go

@@ -608,7 +608,7 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *ast.TypeSpec, def *TypeName
 			Unalias(alias) // resolve alias.actual
 		} else {
 			if !versionErr && tparam0 != nil {
-				check.error(tdecl, UnsupportedFeature, "generic type alias requires GODEBUG=gotypesalias=1")
+				check.error(tdecl, UnsupportedFeature, "generic type alias requires GODEBUG=gotypesalias=1 or unset")
 				versionErr = true
 			}
 

src/go/types/eval_test.go

@@ -178,8 +178,11 @@ func TestEvalPos(t *testing.T) {
 		// Materialized aliases give a different (better)
 		// result for the final test, so skip it for now.
 		// TODO(adonovan): reenable when gotypesalias=1 is the default.
-		if gotypesalias.Value() == "1" && strings.Contains(src, "interface{R}.Read") {
-			continue
+		switch gotypesalias.Value() {
+		case "", "1":
+			if strings.Contains(src, "interface{R}.Read") {
+				continue
+			}
 		}
 
 		files = append(files, file)

src/go/types/issues_test.go

@@ -998,7 +998,7 @@ type A = []int
 type S struct{ A }
 `
 
-	t.Setenv("GODEBUG", "gotypesalias=1")
+	// t.Setenv("GODEBUG", "gotypesalias=1") // now on by default
 	pkg := mustTypecheck(src, nil, nil)
 
 	S := pkg.Scope().Lookup("S")

src/internal/godebugs/table.go

@@ -30,7 +30,7 @@ var All = []Info{
 	{Name: "gocachehash", Package: "cmd/go"},
 	{Name: "gocachetest", Package: "cmd/go"},
 	{Name: "gocacheverify", Package: "cmd/go"},
-	{Name: "gotypesalias", Package: "go/types", Opaque: true}, // bug #66216: remove Opaque
+	{Name: "gotypesalias", Package: "go/types", Changed: 23, Old: "0", Opaque: true}, // bug #66216: remove Opaque
 	{Name: "http2client", Package: "net/http"},
 	{Name: "http2debug", Package: "net/http", Opaque: true},
 	{Name: "http2server", Package: "net/http"},