cgen: fix call arg type changing on match expr (#16744)

vlib/v/gen/c/fn.v

@@ -1719,8 +1719,7 @@ fn (mut g Gen) call_args(node ast.CallExpr) {
 					exp_sym := g.table.sym(expected_types[i])
 					orig_sym := g.table.sym(arg.expr.obj.orig_type)
 					if orig_sym.kind != .interface_ && (exp_sym.kind != .sum_type
-						|| (exp_sym.kind == .sum_type
-						&& expected_types[i] != arg.expr.obj.orig_type)) {
+						&& expected_types[i] != arg.expr.obj.orig_type) {
 						expected_types[i] = g.unwrap_generic(arg.expr.obj.smartcasts.last())
 						cast_sym := g.table.sym(expected_types[i])
 						if cast_sym.info is ast.Aggregate {

vlib/v/tests/sumtype_on_match_test.v

@@ -0,0 +1,35 @@
+struct MotionEvent {}
+
+struct ButtonEvent {}
+
+struct WheelEvent {}
+
+type Event = ButtonEvent | MotionEvent | WheelEvent
+
+type GameEvent = ButtonEvent | MotionEvent
+
+fn test_main() {
+	be := ButtonEvent{}
+	assert handle_event(be) == true
+	e := Event(be)
+	assert handle_event(e) == true
+	match e {
+		MotionEvent {
+			assert handle_game_event(e) == true
+		}
+		ButtonEvent {
+			assert handle_game_event(e) == true
+		}
+		else {}
+	}
+}
+
+fn handle_game_event(ge GameEvent) bool {
+	is_button_event := ge is ButtonEvent
+	return is_button_event
+}
+
+fn handle_event(e Event) bool {
+	is_button_event := e is ButtonEvent
+	return is_button_event
+}