Object typehint & return type variance

object-variance.php

<?php

class A {}

class B {
    function foo(object $a) : object {
        return $a;
    }
}
class C extends B {
    function foo(A $a) : A {
        return $a;
    }
} // Fatal error: Declaration of C::foo(A $a): A must be compatible with B::foo(object $a): object in object-variance.php on line 14

$a = new A();

$b = new B();
$b->foo($a);

$c = new C();
$c->foo($a);