Revert "go/types, types2: track gotypesalias non-default behavior"

This reverts commit c51f6c62576ab3088a981b9cef90e413ac99e333.

Reason for revert: This breaks toolchain bootstrapping in Google. Root cause investigation is pending.

Error message is:

```
<unknown line number>: internal compiler error: panic: godebug: Value of name not listed in godebugs.All: gotypesalias
```

Change-Id: Ie3dff566a29b3b0846ebc8fe0a371c656a043a4c
Reviewed-on: https://go-review.googlesource.com/c/go/+/579575
Reviewed-by: Chressie Himpel <chressie@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Lasse Folger <lassefolger@google.com>

8 files changed, 4 insertions, 40 deletions

diff --git a/src/cmd/compile/internal/types2/check.go b/src/cmd/compile/internal/types2/check.go
index 17d1577ee1..cc723f2012 100644
--- a/src/cmd/compile/internal/types2/check.go
+++ b/src/cmd/compile/internal/types2/check.go
@@ -102,12 +102,6 @@ type Checker struct {
 	// If enableAlias is set, alias declarations produce an Alias type.
 	// Otherwise the alias information is only in the type name, which
 	// points directly to the actual (aliased) type.
-	// Starting with Go 1.23, enableAlias is set by default.
-	// Non-default behavior is tracked with gotypesalias.IncNonDefault()
-	// for each declaration of an alias type where enableAlias is not set.
-	//
-	// TODO(gri) Testing runs tests in both modes. Do we need to exclude
-	//           tracking of non-default behavior for tests?
 	enableAlias bool
 
 	conf *Config
diff --git a/src/cmd/compile/internal/types2/check_test.go b/src/cmd/compile/internal/types2/check_test.go
index f1b520d33c..066218772e 100644
--- a/src/cmd/compile/internal/types2/check_test.go
+++ b/src/cmd/compile/internal/types2/check_test.go
@@ -122,7 +122,7 @@ 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, colDelta uint, manual bool, opts ...func(*Config)) {
-	// Alias types are enabled by default.
+	// Alias types are enabled by default
 	testFilesImpl(t, filenames, srcs, colDelta, manual, opts...)
 	if !manual {
 		t.Setenv("GODEBUG", "gotypesalias=0")
diff --git a/src/cmd/compile/internal/types2/decl.go b/src/cmd/compile/internal/types2/decl.go
index fff2da4953..26ce49d87a 100644
--- a/src/cmd/compile/internal/types2/decl.go
+++ b/src/cmd/compile/internal/types2/decl.go
@@ -532,22 +532,6 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *TypeN
 			alias.fromRHS = rhs
 			Unalias(alias) // resolve alias.actual
 		} else {
-			// With Go1.23, the default behavior is to use Alias nodes,
-			// reflected by check.enableAlias. Signal non-default behavior
-			// by calling gotypesalias.IncNonDefault().
-			//
-			// Note: As of Go 1.23, Settings.IncNonDefault is not present
-			// in internal/godebug/godebug.go used during bootstrapping,
-			// only after the tool chain is built and recompiles itself.
-			// Check dynamically for the presence of IncNonDefault.
-			// (This is not an issue for go/types because it is not used
-			// during bootstrap.)
-			//
-			// TODO(gri) replace with direct call when we bootstrap with Go 1.20
-			if s, ok := any(gotypesalias).(interface{ IncNonDefault() }); ok {
-				s.IncNonDefault()
-			}
-
 			if !versionErr && tparam0 != nil {
 				check.error(tdecl, UnsupportedFeature, "generic type alias requires GODEBUG=gotypesalias=1 or unset")
 				versionErr = true
diff --git a/src/go/types/check.go b/src/go/types/check.go
index c651e57fa9..be990eabfe 100644
--- a/src/go/types/check.go
+++ b/src/go/types/check.go
@@ -102,15 +102,9 @@ type Checker struct {
 	// package information
 	// (initialized by NewChecker, valid for the life-time of checker)
 
-	// If enableAlias is set, alias declarations produce an Alias type.
+	// If EnableAlias is set, alias declarations produce an Alias type.
 	// Otherwise the alias information is only in the type name, which
 	// points directly to the actual (aliased) type.
-	// Starting with Go 1.23, enableAlias is set by default.
-	// Non-default behavior is tracked with gotypesalias.IncNonDefault()
-	// for each declaration of an alias type where enableAlias is not set.
-	//
-	// TODO(gri) Testing runs tests in both modes. Do we need to exclude
-	//           tracking of non-default behavior for tests?
 	enableAlias bool
 
 	conf *Config
diff --git a/src/go/types/check_test.go b/src/go/types/check_test.go
index 068507dade..6ad7ef3a27 100644
--- a/src/go/types/check_test.go
+++ b/src/go/types/check_test.go
@@ -134,7 +134,7 @@ 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.
+	// Alias types are enabled by default
 	testFilesImpl(t, filenames, srcs, manual, opts...)
 	if !manual {
 		t.Setenv("GODEBUG", "gotypesalias=0")
diff --git a/src/go/types/decl.go b/src/go/types/decl.go
index 3dab94aa0f..937163fc75 100644
--- a/src/go/types/decl.go
+++ b/src/go/types/decl.go
@@ -607,10 +607,6 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *ast.TypeSpec, def *TypeName
 			alias.fromRHS = rhs
 			Unalias(alias) // resolve alias.actual
 		} else {
-			// With Go1.23, the default behavior is to use Alias nodes,
-			// reflected by check.enableAlias. Signal non-default behavior.
-			gotypesalias.IncNonDefault()
-
 			if !versionErr && tparam0 != nil {
 				check.error(tdecl, UnsupportedFeature, "generic type alias requires GODEBUG=gotypesalias=1 or unset")
 				versionErr = true
diff --git a/src/internal/godebugs/table.go b/src/internal/godebugs/table.go
index a95c8f2f94..e9e043df4c 100644
--- a/src/internal/godebugs/table.go
+++ b/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", Changed: 23, Old: "0"},
+	{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"},
diff --git a/src/runtime/metrics/doc.go b/src/runtime/metrics/doc.go
index fbbeb1a475..deb993241d 100644
--- a/src/runtime/metrics/doc.go
+++ b/src/runtime/metrics/doc.go
@@ -246,10 +246,6 @@ Below is the full list of supported metrics, ordered lexicographically.
 		The number of non-default behaviors executed by the cmd/go
 		package due to a non-default GODEBUG=gocacheverify=... setting.
 
-	/godebug/non-default-behavior/gotypesalias:events
-		The number of non-default behaviors executed by the go/types
-		package due to a non-default GODEBUG=gotypesalias=... setting.
-
 	/godebug/non-default-behavior/http2client:events
 		The number of non-default behaviors executed by the net/http
 		package due to a non-default GODEBUG=http2client=... setting.