hphp/hack/test/typecheck/require_extend_class_with_incompatible_instantiations_of_invariant_interface.php

   1 <?hh // strict
   2
   3 interface I<T> {
   4   public function get(): T;
   5 }
   6
   7 class A implements I<int>, I<num> {
   8   public function get(): int {
   9     return 0;
  10   }
  11 }
  12
  13 trait T {
  14   require extends A;
  15 }
  16
  17 class B extends A {
  18   use T;
  19   // Using T here is not an error, but a bug in Decl_linearize caused us to
  20   // consider it to be one. When checking that B satisfies its requirements,
  21   // instead of verifying only that B extends A, we verified that B was
  22   // compatibile with A and *also* all of A's ancestors (I<int> and I<num>).
  23   // When a class like A implements a generic interface with multiple
  24   // parameterizations, we arbitrarily select the first as canonical, so A and B
  25   // implement I<int> and NOT I<num> from the perspective of the typechecker.
  26   // Since I is invariant in its type parameter, when we attempt to verify that
  27   // B <: I<num>, we emit an error.
  28
  29   // We should probably not allow classes like A, but we seem to have many in
  30   // practice.
  31 }