While trying to replace typing._GenericAlias with GenericAlias I have found that the latter does not support nested type variables. >>> from typing import * >>> T = TypeVar('T') >>> X = List[List[T]] >>> X.__parameters__ (~T,) >>> X[int] typing.List[typing.List[int]] >>> Y = list[list[T]] >>> Y.__parameters__ () >>> Y[int] Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: There are no type variables left in list[list[~T]]

Good catch. Is there a reasonable fix?

Not yet. If you don't prefer to fix it yourself, I'll do it as soon as I have time.

There is a difference between PR 19836 and the typing module in handling nested unsubscribed generic aliases: >>> from typing import * >>> T = TypeVar('T') >>> D1 = Dict[T, List] >>> D2 = dict[T, List] >>> D1.__parameters__ (~T,) >>> D1[int] typing.Dict[int, typing.List[~T]] >>> D1[int].__parameters__ (~T,) >>> D1[int][str] typing.Dict[int, typing.List[str]] >>> D1[int, str] Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/home/serhiy/py/cpython/Lib/typing.py", line 267, in inner return func(*args, **kwds) File "/home/serhiy/py/cpython/Lib/typing.py", line 686, in __getitem__ _check_generic(self, params) File "/home/serhiy/py/cpython/Lib/typing.py", line 221, in _check_generic raise TypeError(f"Too {'many' if alen > elen else 'few'} parameters for {cls};" TypeError: Too many parameters for typing.Dict[~T, typing.List]; actual 2, expected 1 >>> D2.__parameters__ (~T, ~T) >>> D2[int] Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: Too few arguments for dict[~T, typing.List] >>> D2[int, str] dict[int, typing.List[str]] But this behavior is not specified and is not covered by tests.

 New changeset 41a64587a0fd68bcd21ba42999cd3940801dff7c by Serhiy Storchaka in branch 'master': bpo-40408: Fix support of nested type variables in GenericAlias. (GH-19836) https://github.com/python/cpython/commit/41a64587a0fd68bcd21ba42999cd3940801dff7c 

> But this behavior is not specified and is not covered by tests. FWIW, to be most close to the static type checkers behavior, both D[int][str] and D[int, str] should fail for D = Dict[T, List]. Not important however, since this is a really rare corner case I think.