go/types, types2: coreType/String must consider Alias types

Fixes regression from Go 1.22.

For #67547.

Change-Id: Idd319b9d2a73c824caa2c821df0e2fcd4f58cb08
Reviewed-on: https://go-review.googlesource.com/c/go/+/587176
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: griesemer

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

@@ -22,6 +22,7 @@ func under(t Type) Type {
 // identical element types), the single underlying type is the restricted
 // channel type if the restrictions are always the same, or nil otherwise.
 func coreType(t Type) Type {
+	t = Unalias(t)
 	tpar, _ := t.(*TypeParam)
 	if tpar == nil {
 		return under(t)
@@ -51,6 +52,7 @@ func coreType(t Type) Type {
 // and strings as identical. In this case, if successful and we saw
 // a string, the result is of type (possibly untyped) string.
 func coreString(t Type) Type {
+	t = Unalias(t)
 	tpar, _ := t.(*TypeParam)
 	if tpar == nil {
 		return under(t) // string or untyped string

src/go/types/under.go

@@ -8,3 +8,15 @@ func _[P int]() {
 	type A = P
 	_ = A(0) // don't crash with this conversion
 }
+
+func _[P []int]() {
+	type A = P
+	_ = make(A, 10) // don't report an error for A
+}
+
+func _[P string]() {
+	var t []byte
+	type A = P
+	var s A
+	copy(t, s) // don't report an error for s
+}