go/types, types2: operand.AssignableTo must consider Alias types

Fixes regression from Go 1.22.

For #67547.

Change-Id: Id0d07d6b24e1eab6ed1c7476d9d9b82d28aee80a
Reviewed-on: https://go-review.googlesource.com/c/go/+/587161
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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

@@ -260,7 +260,9 @@ func (x *operand) assignableTo(check *Checker, T Type, cause *string) (bool, Cod
 		return true, 0 // avoid spurious errors
 	}
 
-	V := x.typ
+	origT := T
+	V := Unalias(x.typ)
+	T = Unalias(T)
 
 	// x's type is identical to T
 	if Identical(V, T) {
@@ -386,7 +388,7 @@ func (x *operand) assignableTo(check *Checker, T Type, cause *string) (bool, Cod
 			x.typ = V.typ
 			ok, code = x.assignableTo(check, T, cause)
 			if !ok {
-				errorf("cannot assign %s (in %s) to %s", V.typ, Vp, T)
+				errorf("cannot assign %s (in %s) to %s", V.typ, Vp, origT)
 				return false
 			}
 			return true

src/internal/types/testdata/fixedbugs/issue67547.go

@@ -53,7 +53,28 @@ func _[P int | string]() {
 	// preserve target type name A in error messages when using Alias types
 	// (test are run with and without Alias types enabled, so we need to
 	// keep both A and int in the error message)
-	_ = A(p /* ERRORx "cannot convert string .* to type (A|int)" */)
+	_ = A(p /* ERRORx `cannot convert string \(in P\) to type (A|int)` */)
+}
+
+func _[P struct{ x int }]() {
+	var x struct{ x int }
+	type A = P
+	var _ A = x // assignment must be valid
+}
+
+func _[P struct{ x int }]() {
+	type A = P
+	var x A
+	var _ struct{ x int } = x // assignment must be valid
+}
+
+func _[P []int | struct{}]() {
+	type A = []int
+	var a A
+	var p P
+	// preserve target type name A in error messages when using Alias types
+	a = p // ERRORx `cannot assign struct{} \(in P\) to (A|\[\]int)`
+	_ = a
 }
 
 // Test case for go.dev/issue/67540.