Hm, let me back-pedal a bit. The situation with Callable is murky, as e.g. isinstance(typing.Tuple[int, int], typing.Callable) returns True.

Maybe we need to take a step back and look at the needs for code that wants to implement runtime type checking more in general? ISTM we have ways to access the parameters of a parameterized type (typically t.__parameters__) but we don't have a reasonable way yet to determine what kind of thing t is.