Defining a union of typing.re.Pattern and anything that isn't AnyStr raises a TypeError exception.

Example:

from typing import Union, re

def foo(pattern: Union[str, re.Pattern]) -> None:
    pass

Exception traceback:

Traceback (most recent call last):
  File "/tmp/testtyping.py", line 7, in <module>
    def foo(pattern: Union[str, re.Pattern]) -> None:
  File "/usr/local/lib/python3.5/typing.py", line 534, in __getitem__
    dict(self.__dict__), parameters, _root=True)
  File "/usr/local/lib/python3.5/typing.py", line 491, in __new__
    for t2 in all_params - {t1} if not isinstance(t2, TypeVar)):
  File "/usr/local/lib/python3.5/typing.py", line 491, in <genexpr>
    for t2 in all_params - {t1} if not isinstance(t2, TypeVar)):
TypeError: issubclass() arg 1 must be a class

Confirmed. The failure is because Pattern is _TypeAlias, which is not a type. But it should be allowed. I'll think of something. (Does mypy accept this?)

New changeset 955d3faf727a by Guido van Rossum in branch '3.5':
Issue #25390: typing: Don't crash on Union[str, Pattern].
https://hg.python.org/cpython/rev/955d3faf727a

New changeset 04314479af0b by Guido van Rossum in branch 'default':
Issue #25390: typing: Don't crash on Union[str, Pattern]. (Merge 3.5->3.6)
https://hg.python.org/cpython/rev/04314479af0b