>>> from typing import *
>>> get_args(Callable)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/typing.py", line 1412, in get_args
    if get_origin(tp) is collections.abc.Callable and res[0] is not Ellipsis:
IndexError: tuple index out of range

What is the correct behavior?

Maybe it could return (Ellipsis, Any) in that case. The most general form of Callable is Callable[..., Any] and that's that get_args() returns for that.

I think it should return empty tuple: First, for consistency with other things like get_args(Tuple) == () and get_args(List) == (). Second, although Callable is equivalent to Callable[..., Any] in the static world, authors of some runtime checkers might want to distinguish whether a user wrote one or another.

Agreed, that's better!

get_args(List) == (T,)

Oh my, this looks like another bug. I don't have Python 3.8 at hand so just tried `typing_inspect` on Python 3.6. I think this may be caused by the "double use" of _GenericAlias for which you opened your WIP PR.

New changeset 6292be7adf247589bbf03524f8883cb4cb61f3e9 by Serhiy Storchaka in branch 'master':
bpo-40398: Fix typing.get_args() for special generic aliases. (GH-19720)
https://github.com/python/cpython/commit/6292be7adf247589bbf03524f8883cb4cb61f3e9

@Serhiy is this fixed by PR 19720?

Yes, I have not closed this issue yet because it required a manual backporting to 3.8.

New changeset a629d4c63c55ba36be36ff105dfc103b710c9a2d by Serhiy Storchaka in branch '3.8':
[3.8] bpo-40398: Fix typing.get_args() for special generic aliases. (GH-19720) (GH-19857)
https://github.com/python/cpython/commit/a629d4c63c55ba36be36ff105dfc103b710c9a2d