typing.Union unifies types too broadly

[agronholm]

BPO	26075
Nosy	@gvanrossum, @ezio-melotti, @agronholm, @ambv, @ilevkivskyi

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2016-09-27.22:25:19.274>
created_at = <Date 2016-01-10.12:57:34.647>
labels = ['extension-modules', 'type-bug']
title = 'typing.Union unifies types too broadly'
updated_at = <Date 2016-09-27.22:25:19.273>
user = 'https://github.com/agronholm'

bugs.python.org fields:

activity = <Date 2016-09-27.22:25:19.273>
actor = 'gvanrossum'
assignee = 'none'
closed = True
closed_date = <Date 2016-09-27.22:25:19.274>
closer = 'gvanrossum'
components = ['Extension Modules']
creation = <Date 2016-01-10.12:57:34.647>
creator = 'alex.gronholm'
dependencies = []
files = []
hgrepos = []
issue_num = 26075
keywords = []
message_count = 3.0
messages = ['257910', '257925', '277559']
nosy_count = 5.0
nosy_names = ['gvanrossum', 'ezio.melotti', 'alex.gronholm', 'lukasz.langa', 'levkivskyi']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'test needed'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue26075'
versions = ['Python 3.5', 'Python 3.6']

[agronholm]

>>> from typing import Union, Iterable
>>> Union[str, Iterable[int]]
typing.Iterable[int]

The union loses the "str" parameter because issubclass(str, collections.abc.Iterable) returns True and the check completely disregards generics.

Guido mentioned that issubclass() support for generic types should be going away. In the mean time, maybe the subclass check in typing.GenericMeta should be modified not to do it with types from Typing, but how can we accurately identify them?

[gvanrossum]

Oh, this is yet another reason to finally rip out the issubclass() support... We're tracking that at python/typing#136

In the meantime, just don't do this!

[gvanrossum]

Fixed by 09cc43df4509.