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;
  }
}

Output:

$ hh_client main.hh
main.hh:17:5,27: Invalid assignment (Typing[4110])
  main.hh:3:15,18: This is a value of generic type Tval
  main.hh:4:11,14:   via this generic Tval
  main.hh:16:23,25: It is incompatible with a num (int/float)

HHVM version:

$ hhvm --version
HipHop VM 3.8.0-dev (rel)
Compiler: heads/master-0-g1100e471847b144850917aea3fda0dba4b86d99b
Repo schema: 9ffd6c11e5c199737209a7d4ee4e10a25fbc3f94

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