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

New changeset e082e7cbe4a934b86f7a07354d97d4e14a9dd46a by Ivan Levkivskyi (plokmijnuhby) in branch 'master':
bpo-37953: Fix ForwardRef hash and equality checks (GH-15400)
https://github.com/python/cpython/commit/e082e7cbe4a934b86f7a07354d97d4e14a9dd46a

New changeset e91edfed4214dbae17b8906b5dc7778769aac620 by Miss Islington (bot) in branch '3.8':
bpo-37953: Fix ForwardRef hash and equality checks (GH-15400)
https://github.com/python/cpython/commit/e91edfed4214dbae17b8906b5dc7778769aac620

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.

New changeset d057b896f97e6d7447b9bf9246770c41cf205299 by Miss Islington (bot) (Zackery Spytz) in branch 'master':
bpo-37953: Fix deprecation warnings in test_typing (GH-16133)
https://github.com/python/cpython/commit/d057b896f97e6d7447b9bf9246770c41cf205299

New changeset 66da347ef0034ad9bddc7fad112025c886249f0d by Miss Islington (bot) in branch '3.8':
bpo-37953: Fix deprecation warnings in test_typing (GH-16133)
https://github.com/python/cpython/commit/66da347ef0034ad9bddc7fad112025c886249f0d

New changeset 3eff46fc7d2e3c80c4dedba4177782f1fc8ad89b by Ryan Rowe in branch '3.7':
bpo-37953: Fix ForwardRef hash and equality checks (GH-15400) (GH-18751)
https://github.com/python/cpython/commit/3eff46fc7d2e3c80c4dedba4177782f1fc8ad89b