Why `this` in a contravariant position in an interface doesn't work out

CovariantThisInInterface.php

<?hh // strict
interface Base {
	public function foo(this $v): void;
}
final class Derived<T> implements Base {
	public function bar(): void {}
	public function foo(this $v): void {
		$v->bar(); // an `OtherDerived` here would be unwelcome.
	}
}
final class OtherDerived implements Base {
	public function foo(this $v): void {}
}
function violate(Base $v, Base $x): void {
	$v->foo($x);
}

/* HH_FIXME[1002] Try to violate in strict mode */
violate(new Derived(), new OtherDerived());