go/types, types2: don't panic converting a constant to aliased type parameter

For #67547.

Change-Id: I1b2118a311dce906327ae6e29e582da539c60b2b
Reviewed-on: https://go-review.googlesource.com/c/go/+/587157
Auto-Submit: Robert Griesemer <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: griesemer

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

@@ -56,7 +56,7 @@ func (check *Checker) conversion(x *operand, T Type) {
 		// If T's type set is empty, or if it doesn't
 		// have specific types, constant x cannot be
 		// converted.
-		ok = T.(*TypeParam).underIs(func(u Type) bool {
+		ok = Unalias(T).(*TypeParam).underIs(func(u Type) bool {
 			// u is nil if there are no specific type terms
 			if u == nil {
 				cause = check.sprintf("%s does not contain specific types", T)

src/go/types/conversions.go

@@ -0,0 +1,10 @@
+// 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
+
+func _[P int]() {
+	type A = P
+	_ = A(0) // don't crash with this conversion
+}