cmd/compile: support generic alias type

Type parameters on aliases are now allowed after #46477 accepted.

Updates #46477
Fixes #68054

Change-Id: Ic2e3b6f960a898163f47666e3a6bfe43b8cc22e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/593715


Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@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/noder/writer.go

@@ -543,7 +543,7 @@ func (pw *pkgWriter) typIdx(typ types2.Type, dict *writerDict) typeInfo {
 
 	case *types2.Alias:
 		w.Code(pkgbits.TypeNamed)
-		w.namedType(typ.Obj(), nil)
+		w.namedType(splitAlias(typ))
 
 	case *types2.TypeParam:
 		w.derived = true
@@ -2958,6 +2958,9 @@ func objTypeParams(obj types2.Object) *types2.TypeParamList {
 		if !obj.IsAlias() {
 			return obj.Type().(*types2.Named).TypeParams()
 		}
+		if alias, ok := obj.Type().(*types2.Alias); ok {
+			return alias.TypeParams()
+		}
 	}
 	return nil
 }
@@ -2974,6 +2977,14 @@ func splitNamed(typ *types2.Named) (*types2.TypeName, *types2.TypeList) {
 	return typ.Obj(), typ.TypeArgs()
 }
 
+// splitAlias is like splitNamed, but for an alias type.
+func splitAlias(typ *types2.Alias) (*types2.TypeName, *types2.TypeList) {
+	orig := typ.Origin()
+	base.Assertf(typ.Obj() == orig.Obj(), "alias type %v has object %v, but %v has object %v", typ, typ.Obj(), orig, orig.Obj())
+
+	return typ.Obj(), typ.TypeArgs()
+}
+
 func asPragmaFlag(p syntax.Pragma) ir.PragmaFlag {
 	if p == nil {
 		return 0

test/fixedbugs/issue68054.go

+// compile -goexperiment aliastypeparams
+
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type Seq[V any] = func(yield func(V) bool)
+
+func f[E any](seq Seq[E]) {
+	return
+}
+
+func g() {
+	f(Seq[int](nil))
+}
+
+type T[P any] struct{}
+
+type A[P any] = T[P]
+
+var _ A[int]