async def psubscribe(self, *args: ChannelT, **kwargs: PubSubHandler):
        new_patterns: Dict[ChannelT, Optional[PubSubHandler]] = dict.fromkeys(args)
        new_patterns.update(kwargs)

This code (taken from aioredis) produces this error:
error: Argument 1 to "update" of "dict" has incompatible type "Dict[str, Callable[[Dict[str, str]], None]]"; expected "Mapping[Union[bytes, str, memoryview], Optional[Callable[[Dict[str, str]], None]]]" [arg-type]

But, the types are compatible as the keys and values are part of the expected union, I can't think of any operation that would make this unsafe. I understand it could be unsafe if the expectation was MutableMapping, as the function might assign any of the unioned types, but for Mapping this looks completely safe to me.