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]]