I encountered another bug, the __repr__ needs to be updated as well:

>>> ca.Callable[[int, P], int][P][int]
collections.abc.Callable[[int, (<class 'int'>,)], int]

EDIT: THIS MIGHT NOT BE A BUG, AS ITS AN INVALID USE. PLEASE IGNORE THIS @sobolevn

@Fidget-Spinner I would like to move all repr related problems to another PR if that possible :)

I've only touched params substitution here.

Wait a moment, this is not about repr, this is an __args__ containing a tuple 😒

>>> ca.Callable[[int, P], int][P].__args__
(<class 'int'>, ~P, <class 'int'>)
>>> ca.Callable[[int, P], int][P][int].__args__
(<class 'int'>, (<class 'int'>,), <class 'int'>)

Funny thing, I just removed this code from _collections_abc.py as an experiment:

if (len(self.__parameters__) == 1
                and _is_param_expr(self.__parameters__[0])
                and item and not _is_param_expr(item[0])):
            item = (item,)

All tests still pass :)

🤯

I am going to close this for now, I will add new tests in other PRs, because this one has a big unrelated history.