This is a feature request for a feature I don't think many are interested in. Still, worth thinking about.

class X {
  /** @var ?int **/
  private $x;

  public function getX(): int {
    $this->x = 5;

    $this->modifyX();

    return $this->x;
  }

  private function modifyX(): void {
    $this->x = null;
  }
}

Hack forbids the above code (though to see it you have to change the property type to one hack understands). Psalm, by default, allows the above code, but a config flag makes it follow Hack's behaviour.