_TypeAlias: Discrepancy between docstring and behavior

[flying-sheep]

BPO	25830
Nosy	@gvanrossum, @flying-sheep, @berkerpeksag, @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 2016-09-27.22:26:00.378>
created_at = <Date 2015-12-09.11:57:32.831>
labels = ['type-bug', 'library']
title = '_TypeAlias: Discrepancy between docstring and behavior'
updated_at = <Date 2016-09-27.22:26:00.377>
user = 'https://github.com/flying-sheep'

bugs.python.org fields:

activity = <Date 2016-09-27.22:26:00.377>
actor = 'gvanrossum'
assignee = 'none'
closed = True
closed_date = <Date 2016-09-27.22:26:00.378>
closer = 'gvanrossum'
components = ['Library (Lib)']
creation = <Date 2015-12-09.11:57:32.831>
creator = 'flying sheep'
dependencies = []
files = []
hgrepos = []
issue_num = 25830
keywords = []
message_count = 4.0
messages = ['256154', '258776', '258778', '277560']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'flying sheep', 'berker.peksag', 'levkivskyi']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue25830'
versions = ['Python 3.5', 'Python 3.6']

[flying-sheep]

_TypeAlias claims in its docstring that “It can be used in instance and subclass checks”, yet promptly contradicts itself if you try it: “Type aliases cannot be used with isinstance().”

it would be useful to either document (and therefore bless) type_impl, or make it actually work with isinstance (probably by delegating to type_impl)

[berkerpeksag]

Looks like the docstring is a bit misleading. isinstance(m, Match) behavior is already tested in the test suite: https://hg.python.org/cpython/file/default/Lib/test/test_typing.py#l1297

[gvanrossum]

Actually isinstance() shouldn't be supported by most things in typing.py
(except for those that mimic existing collection ABCs). Even issubclass()
should be taken out according to the BDFL-delegate for PEP-484. See
python/typing#136

[gvanrossum]

Fixed by 09cc43df4509.