Fix ForwardRef equality checks

[plokmijnuhby]

BPO	37953
Nosy	@ned-deily, @ilevkivskyi, @ZackerySpytz, @miss-islington, @plokmijnuhby
PRs	bpo-37953: Fix ForwardRef equality checks #15400 bpo-37953:Improve ForwardRef equality checks #15628 bpo-37953:Improve ForwardRef equality check #15650 [3.8] bpo-37953: Fix ForwardRef hash and equality checks (GH-15400) #16128 bpo-37953: Fix deprecation warnings in test_typing #16133 [3.8] bpo-37953: Fix deprecation warnings in test_typing (GH-16133) #16138 [3.7] bpo-37953: Fix ForwardRef hash and equality checks (GH-15400) #18751

^{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 2019-09-13.20:05:54.656>
created_at = <Date 2019-08-26.14:06:04.423>
labels = ['3.7', '3.8', 'type-bug', 'library', '3.9']
title = 'Fix ForwardRef equality checks'
updated_at = <Date 2020-03-03.22:30:18.088>
user = 'https://github.com/plokmijnuhby'

bugs.python.org fields:

activity = <Date 2020-03-03.22:30:18.088>
actor = 'ned.deily'
assignee = 'none'
closed = True
closed_date = <Date 2019-09-13.20:05:54.656>
closer = 'levkivskyi'
components = ['Library (Lib)']
creation = <Date 2019-08-26.14:06:04.423>
creator = 'plokmijnuhby'
dependencies = []
files = []
hgrepos = []
issue_num = 37953
keywords = ['patch']
message_count = 7.0
messages = ['350531', '352389', '352391', '352413', '352419', '352421', '363313']
nosy_count = 6.0
nosy_names = ['ned.deily', 'python-dev', 'levkivskyi', 'ZackerySpytz', 'miss-islington', 'plokmijnuhby']
pr_nums = ['15400', '15628', '15650', '16128', '16133', '16138', '18751']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue37953'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

[plokmijnuhby]

Apologies for issuing a pull request without an associated issue. I'm kind of new to this. Nevermind, I'm making one now.

The typing module currently contains a bug where ForwardRefs change their hash and equality once they are evaluated. Consider the following code:

import typing
ref = typing.ForwardRef('MyClass')
ref_ = typing.ForwardRef('MyClass')


class MyClass:
    def __add__(self, other: ref): ...


# We evaluate one forward reference, but not the other.
typing.get_type_hints(MyClass.__add__)

# Equality is violated
print(ref == ref_) # False

# This can cause duplication in Unions.
# The following prints:
# typing.Union[ForwardRef('MyClass'), ForwardRef('MyClass')]
# when it should be: typing.Union[ForwardRef('MyClass')]
wrong = typing.Union[ref, ref_]
print(wrong)

# The union also does not compare equality properly
should_be_equal = typing.Union[ref]
print(should_be_equal == wrong) # False

# In fact this applies to any generic
print(typing.Callable[[ref],None] == typing.Callable[[ref_],None]) # False

[ilevkivskyi]

New changeset e082e7c by Ivan Levkivskyi (plokmijnuhby) in branch 'master':
bpo-37953: Fix ForwardRef hash and equality checks (GH-15400)
e082e7c

[miss-islington]

New changeset e91edfe by Miss Islington (bot) in branch '3.8':
bpo-37953: Fix ForwardRef hash and equality checks (GH-15400)
e91edfe

[ZackerySpytz]

There are deprecation warnings in test_typing.

./python -m test test_typing
Run tests sequentially
0:00:00 load avg: 0.16 [1/1] test_typing
/home/lubuntu2/cpython/Lib/test/test_typing.py:2382: DeprecationWarning: Please use assertEqual instead.
self.assertEquals(Union[c1, c1_gth], Union[c1])
/home/lubuntu2/cpython/Lib/test/test_typing.py:2383: DeprecationWarning: Please use assertEqual instead.
self.assertEquals(Union[c1, c1_gth, int], Union[c1, int])

== Tests result: SUCCESS ==

1 test OK.

Total duration: 215 ms
Tests result: SUCCESS

I've created PR 16133 to fix them.

[miss-islington]

New changeset d057b89 by Miss Islington (bot) (Zackery Spytz) in branch 'master':
bpo-37953: Fix deprecation warnings in test_typing (GH-16133)
d057b89

[miss-islington]

New changeset 66da347 by Miss Islington (bot) in branch '3.8':
bpo-37953: Fix deprecation warnings in test_typing (GH-16133)
66da347

[ned-deily]

New changeset 3eff46f by Ryan Rowe in branch '3.7':
bpo-37953: Fix ForwardRef hash and equality checks (GH-15400) (GH-18751)
3eff46f