class C {
  /**
   * @param mixed $x
   * @param-out string|int $x
   */
  public function foo(&$x): void {
    // Method C::foo() never assigns int to &$x so it can be removed from the @param-out type.
    $x = "foo";
  }
}

class D extends C {
  public function foo(&$x): void {
    // Method D::foo() never assigns string to &$x so it can be removed from the @param-out type.
    $x = 42;
  }
}