add __weakref__ to types.GenericAlias

[Tinche]

BPO	42332
Nosy	@gvanrossum, @Tinche, @miss-islington, @Fidget-Spinner
PRs	bpo-42332: Add weakref slot to types.GenericAlias #23250 [3.9] bpo-42332: Add weakref slot to types.GenericAlias (GH-23250) #23309

^{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 2020-11-17.22:53:26.216>
created_at = <Date 2020-11-12.12:04:09.556>
labels = ['type-feature', '3.9', '3.10']
title = 'add __weakref__ to types.GenericAlias'
updated_at = <Date 2020-11-17.22:53:26.215>
user = 'https://github.com/Tinche'

bugs.python.org fields:

activity = <Date 2020-11-17.22:53:26.215>
actor = 'gvanrossum'
assignee = 'none'
closed = True
closed_date = <Date 2020-11-17.22:53:26.216>
closer = 'gvanrossum'
components = []
creation = <Date 2020-11-12.12:04:09.556>
creator = 'tinchester'
dependencies = []
files = []
hgrepos = []
issue_num = 42332
keywords = ['patch']
message_count = 5.0
messages = ['380816', '380834', '380836', '381060', '381301']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'tinchester', 'miss-islington', 'kj']
pr_nums = ['23250', '23309']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue42332'
versions = ['Python 3.9', 'Python 3.10']

[Tinche]

For example, dict[int, int] cannot be used with singledispatch because types.GenericAlias doesn't support weak references.

I'm an author of a third party library (https://github.com/Tinche/cattrs) for which this functionality would be useful.

Here's a similar issue in typing (note that this issue is for *types*.GenericAlias) that was fixed: python/typing#345

[Tinche]

It would be great if we could get this into 3.9.

[gvanrossum]

I think it's reasonable to consider this a bug to be fixed.

[miss-islington]

New changeset 384b7a4 by kj in branch 'master':
bpo-42332: Add weakref slot to types.GenericAlias (GH-23250)
384b7a4

[miss-islington]

New changeset 71ba5f5 by kj in branch '3.9':
[3.9] bpo-42332: Add weakref slot to types.GenericAlias (GH-23250) (GH-23309)
71ba5f5