types2: flip the default value of GODEBUG=gotypesalias=1

This CL changes the interpretation of the unset value
of gotypesalias to not equal "0".

This is a port of CL 577715 from go/types to types2,
with adjustments to go/types to keep the source code
in sync. Specifically:

- Re-introduce testing of both modes (gotypesalias=0,
  gotypesalias=1) in go/types.
- Re-introduce setting of gotypesalias in some of the
  tests for explicit documentation in go/types.

The compiler still uses the (now) non-default setting
due to a panic with the default setting that needs to
be debugged.

Also, the type checkers still don't call IncNonDefault
when the non-default setting of gotypesalias is used.

Change-Id: I1feed3eb334c202950ac5aadf49a74adcce0d8c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/579076
TryBot-Bypass: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>

4 files changed, 17 insertions, 10 deletions

diff --git a/src/go/types/api_test.go b/src/go/types/api_test.go
index 7ab695d365..5bc4e8a61f 100644
--- a/src/go/types/api_test.go
+++ b/src/go/types/api_test.go
@@ -2997,6 +2997,7 @@ 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
diff --git a/src/go/types/check.go b/src/go/types/check.go
index 74849171f2..be990eabfe 100644
--- a/src/go/types/check.go
+++ b/src/go/types/check.go
@@ -24,8 +24,9 @@ var noposn = atPos(nopos)
 const debug = false // leave on during development
 
 // gotypesalias controls the use of Alias types.
-// As of Apr 12 2024 it is on by default.
-// It will be removed soon.
+// As of Apr 16 2024 they are used by default.
+// To disable their use, set GODEBUG to gotypesalias=0.
+// This GODEBUG flag will be removed in the near future (tentatively Go 1.24).
 var gotypesalias = godebug.New("gotypesalias")
 
 // exprInfo stores information about an untyped expression.
@@ -260,14 +261,8 @@ 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: enableAlias,
+		enableAlias: gotypesalias.Value() != "0",
 		conf:        conf,
 		ctxt:        conf.Context,
 		fset:        fset,
diff --git a/src/go/types/check_test.go b/src/go/types/check_test.go
index 63891e9056..6ad7ef3a27 100644
--- a/src/go/types/check_test.go
+++ b/src/go/types/check_test.go
@@ -124,6 +124,8 @@ 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
@@ -132,6 +134,15 @@ 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 enabled by default
+	testFilesImpl(t, filenames, srcs, manual, opts...)
+	if !manual {
+		t.Setenv("GODEBUG", "gotypesalias=0")
+		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")
 	}
diff --git a/src/go/types/issues_test.go b/src/go/types/issues_test.go
index eb627eaee7..4f4bf6f077 100644
--- a/src/go/types/issues_test.go
+++ b/src/go/types/issues_test.go
@@ -998,7 +998,7 @@ type A = []int
 type S struct{ A }
 `
 
-	// t.Setenv("GODEBUG", "gotypesalias=1") // now on by default
+	t.Setenv("GODEBUG", "gotypesalias=1")
 	pkg := mustTypecheck(src, nil, nil)
 
 	S := pkg.Scope().Lookup("S")