go/types, types2: use GOEXPERIMENT to enable alias type parameters

For #46477.

Change-Id: Ia3558f9d2bf43fdd9e3618bd9f800d268e13b367
Reviewed-on: https://go-review.googlesource.com/c/go/+/586956
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>

src/cmd/compile/internal/types2/decl.go

@@ -8,6 +8,7 @@ import (
 	"cmd/compile/internal/syntax"
 	"fmt"
 	"go/constant"
+	"internal/buildcfg"
 	. "internal/types/errors"
 )
 
@@ -522,6 +523,10 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *TypeN
 
 			// handle type parameters even if not allowed (Alias type is supported)
 			if tparam0 != nil {
+				if !versionErr && !buildcfg.Experiment.AliasTypeParams {
+					check.error(tdecl, UnsupportedFeature, "generic type alias requires GOEXPERIMENT=aliastypeparams")
+					versionErr = true
+				}
 				check.openScope(tdecl, "type parameters")
 				defer check.closeScope()
 				check.collectTypeParams(&alias.tparams, tdecl.TParamList)

src/go/types/decl.go

@@ -9,6 +9,7 @@ import (
 	"go/ast"
 	"go/constant"
 	"go/token"
+	"internal/buildcfg"
 	. "internal/types/errors"
 )
 
@@ -597,6 +598,10 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *ast.TypeSpec, def *TypeName
 
 			// handle type parameters even if not allowed (Alias type is supported)
 			if tparam0 != nil {
+				if !versionErr && !buildcfg.Experiment.AliasTypeParams {
+					check.error(tdecl, UnsupportedFeature, "generic type alias requires GOEXPERIMENT=aliastypeparams")
+					versionErr = true
+				}
 				check.openScope(tdecl, "type parameters")
 				defer check.closeScope()
 				check.collectTypeParams(&alias.tparams, tdecl.TypeParams)

src/internal/types/testdata/spec/typeAliases1.23b.go

@@ -1,4 +1,4 @@
-// -lang=go1.23 -gotypesalias=1
+// -lang=go1.23 -gotypesalias=1 -goexperiment=aliastypeparams
 
 // Copyright 2024 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style