main.hh

<?hh

class Wrapper<Tval as num> { 
  private Tval $value;

  public function __construct(Tval $value) { 
    $this->value = $value;
  }
  public function setValue(Tval $value): void {
    $this->value = $value;
  }
  public function getValue(): Tval { 
    return $this->value;
  } 

  public function add(num $addend): void {
    $this->value += $addend;
  }
}

http://3v4l.org/Wr33k <- Claims it works in previous versions...