go/types, types2: unalias tilde terms in underIs

Unalias the ~T terms during underIs. Before, if T was an alias
of U, it may pass T to the iteration function. The iterator
function expects an underlying type, under(U), to be passed.
This caused several bugs where underIs is used without
eventually taking the underlying type.

Updates #68935
Fixes #68903

Change-Id: Ie8691d8dddaea00e1dcba94d17c0f1b021fc49a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/606075
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -131,8 +131,8 @@ func (s *_TypeSet) underIs(f func(Type) bool) bool {
 	}
 	for _, t := range s.terms {
 		assert(t.typ != nil)
-		// x == under(x) for ~x terms
-		u := t.typ
+		// Unalias(x) == under(x) for ~x terms
+		u := Unalias(t.typ)
 		if !t.tilde {
 			u = under(u)
 		}

src/go/types/typeset.go

@@ -134,8 +134,8 @@ func (s *_TypeSet) underIs(f func(Type) bool) bool {
 	}
 	for _, t := range s.terms {
 		assert(t.typ != nil)
-		// x == under(x) for ~x terms
-		u := t.typ
+		// Unalias(x) == under(x) for ~x terms
+		u := Unalias(t.typ)
 		if !t.tilde {
 			u = under(u)
 		}

src/internal/types/testdata/fixedbugs/issue68903.go

@@ -0,0 +1,24 @@
+// 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 A = [4]int
+type B = map[string]interface{}
+
+func _[T ~A](x T) {
+	_ = len(x)
+}
+
+func _[U ~A](x U) {
+	_ = cap(x)
+}
+
+func _[V ~A]() {
+	_ = V{}
+}
+
+func _[W ~B](a interface{}) {
+	_ = a.(W)["key"]
+}

src/internal/types/testdata/fixedbugs/issue68935.go

@@ -0,0 +1,26 @@
+// 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 A = struct {
+	F string
+	G int
+}
+
+func Make[T ~A]() T {
+	return T{
+		F: "blah",
+		G: 1234,
+	}
+}
+
+type N struct {
+	F string
+	G int
+}
+
+func _() {
+	_ = Make[N]()
+}