go/types: track gotypesalias non-default behavior

Fixes #66216.

Change-Id: I04d7389e5712b35db078844ce424e10f5b96156c
Reviewed-on: https://go-review.googlesource.com/c/go/+/579936
Auto-Submit: Robert Griesemer <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: griesemer

src/go/types/decl.go

@@ -607,6 +607,13 @@ 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.
+			//
+			// TODO(gri) Testing runs tests in both modes. Do we need to exclude
+			//           tracking of non-default behavior for tests?
+			gotypesalias.IncNonDefault()
+
 			if !versionErr && tparam0 != nil {
 				check.error(tdecl, UnsupportedFeature, "generic type alias requires GODEBUG=gotypesalias=1 or unset")
 				versionErr = true

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", Opaque: true}, // bug #66216: remove Opaque
+	{Name: "gotypesalias", Package: "go/types", Changed: 23, Old: "0"},
 	{Name: "http2client", Package: "net/http"},
 	{Name: "http2debug", Package: "net/http", Opaque: true},
 	{Name: "http2server", Package: "net/http"},

src/runtime/metrics/doc.go

@@ -246,6 +246,10 @@ 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.