pickling fails for typing.Tuple[()]

[ryanthompson591]

Bug report

A clear and concise description of what the bug is.
Include a minimal, reproducible example (https://stackoverflow.com/help/minimal-reproducible-example), if possible.

import pickle
from typing import Tuple

j = Tuple[()]
serialized = pickle.dumps(j)
loaded = pickle.loads(serialized)

Error message:

Traceback (most recent call last):
  File "/Users/ryanthompson/PycharmProjects/learningProjects/dill_regression/regression_test.py", line 6, in <module>
    loaded = dill.loads(serialized)
  File "/Users/ryanthompson/.pyenv/versions/3.8.11/lib/python3.8/site-packages/dill/_dill.py", line 387, in loads
    return load(file, ignore, **kwds)
  File "/Users/ryanthompson/.pyenv/versions/3.8.11/lib/python3.8/site-packages/dill/_dill.py", line 373, in load
    return Unpickler(file, ignore=ignore, **kwds).load()
  File "/Users/ryanthompson/.pyenv/versions/3.8.11/lib/python3.8/site-packages/dill/_dill.py", line 646, in load
    obj = StockUnpickler.load(self)
  File "/Users/ryanthompson/.pyenv/versions/3.8.11/lib/python3.8/typing.py", line 804, in __getitem__
    return self.__getitem_inner__(params)
  File "/Users/ryanthompson/.pyenv/versions/3.8.11/lib/python3.8/typing.py", line 261, in inner
    return func(*args, **kwds)
  File "/Users/ryanthompson/.pyenv/versions/3.8.11/lib/python3.8/typing.py", line 830, in __getitem_inner__
    params = tuple(_type_check(p, msg) for p in params)
  File "/Users/ryanthompson/.pyenv/versions/3.8.11/lib/python3.8/typing.py", line 830, in <genexpr>
    params = tuple(_type_check(p, msg) for p in params)
  File "/Users/ryanthompson/.pyenv/versions/3.8.11/lib/python3.8/typing.py", line 149, in _type_check
    raise TypeError(f"{msg} Got {arg!r:.100}.")
TypeError: Tuple[t0, t1, ...]: each t must be a type. Got ().

Your environment
python 3.8

• CPython versions tested on:
  3.6 & 3.8
• Operating system and architecture:
  Mac OS

[JelleZijlstra]

Thanks for the report. I can reproduce on 3.9 and 3.10, but not a recent build of 3.11. I assume @serhiy-storchaka's changes to the representation of empty tuples fixed this issue.

We're not going to fix this for 3.9 and lower because those branches are in security fix-only mode, but we can still fix it on 3.10.

[mmckerns]

Just for completeness... here's a little more information on the bug (from the OP issue at dill):

>>> import pickle
>>> import typing
>>> t = typing.Tuple[()]
>>> t
typing.Tuple[()]
>>> t = t.copy_with(())
>>> t
typing.Tuple[]
>>> s = pickle.dumps(t)
>>> pickle.loads(s)
typing.Tuple[()]
>>> typing.Tuple[]
  File "<stdin>", line 1
    typing.Tuple[]
                 ^
SyntaxError: invalid syntax

[serhiy-storchaka]

Backporting #31836 can fix this issue. #94260 is an alternate fix. #94259 contains tests, they should be added in all versions.