hphp/hack/test/typecheck/contravariance1.php

   1 <?hh // partial
   2
   3 class Foo<-T> {
   4   public function __construct(T $x) {}
   5 }
   6
   7 class A {}
   8 class B extends A {}
   9
  10 function fa(Foo<A> $x) {}
  11 function fb(Foo<B> $y) {}
  12
  13 function g() {
  14   // $x has type B and therefore also type A
  15   $x = new B();
  16   // $y can be assigned type Foo<A> because B <: A
  17   $y = new Foo($x);
  18   // So this is actually legal
  19   fa($y);
  20   // By contravariance we can also pass it to something expecting Foo<B>
  21   fb($y);
  22 }