Hack typechecker disagrees with run-time re at() signature in ConstIndexAccess

Dummy.hh

<?hh // strict

final class Dummy<Tx> implements ConstIndexAccess<int, Tx>
{
    final public function __construct(private Tx $x) { }

    final public function at(int $ix): Tx
    {
        return $this->x;
    }

    final public function get(int $ix): ?Tx
    {
        return $this->at($ix);
    }

    final public function containsKey<Tu super int>(Tu $ix): bool
    {
        return true;
    }
}

main.hh

<?hh // partial

require_once(__DIR__.'/Dummy.hh');

$d = new Dummy(1);