There's no point in lamenting the compatibility with Python 3.6, it's water under the bridge. Dispatching on types like list[int] or types generated by NewType is not realistic. Maybe the only thing left to do is to raise an error on registration when the type is e.g. list[int]? Currently that passes, but then attempting to dispatch on *anything* fails with TypeError: issubclass() argument 2 cannot be a parameterized generic