hphp/hack/test/typecheck/constraints/super_as_covariant.php

   1 <?hh // strict
   2
   3 class Cov<+T> {}
   4
   5 function UseAs<Tv, Tu as Tv>(Cov<Tv> $x, Cov<Tu> $y): Tu {
   6   //UNSAFE
   7 }
   8
   9 function UseSuper<Tv super Tu, Tu>(Cov<Tv> $x, Cov<Tu> $y): Tu {
  10   //UNSAFE
  11 }
  12
  13 class MyBase {}
  14 class MyDerived extends MyBase {}
  15
  16 function DoesWork(Cov<MyBase> $cb, Cov<MyDerived> $cd): MyDerived {
  17   return UseSuper($cb, $cd);
  18 }
  19
  20 function DoesNotWork(Cov<MyBase> $cb, Cov<MyDerived> $cd): MyDerived {
  21   return UseAs($cb, $cd);
  22 }