Diff from mypy_primer, showing the effect of this PR on open source code:

pyinstrument (https://github.com/joerick/pyinstrument)
- pyinstrument/vendor/decorator.py:279: error: Module "contextlib" has no attribute "GeneratorContextManager"; maybe "_GeneratorContextManager", "ContextManager", or "AbstractContextManager"?
+ pyinstrument/vendor/decorator.py:279: error: Module "contextlib" has no attribute "GeneratorContextManager"; maybe "_GeneratorContextManager", "_AsyncGeneratorContextManager", or "ContextManager"?

There's also a _GeneratorContextManagerBase base class for both _GeneratorContextManager and _AsyncGeneratorContextManager (on Python 3.9), which is worth considering for a future PR.

Well, I had a go. You can see the results in a branch over here -- but I'm not really sure it's worth worrying about, since it seems like it breaks mypy :/

Even after adding 5 # type: ignores I get

stdlib\contextlib.pyi:50: error: INTERNAL ERROR -- Please try using mypy master on Github

when I try to run mypy on it.

Do you have a full stack trace on hand?

For what it's worth, I don't think it's too important to add the _GeneratorContextManagerBase base class since it's private and provides no public methods.

Do you have a full stack trace on hand?

Here's a full traceback:

(.venv3) C:\Users\Alex\Desktop\Code dump\typeshed>mypy test.py --custom-typeshed-dir . --tb
stdlib\contextlib.pyi:49: error: INTERNAL ERROR -- Please try using mypy master on Github:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 0.920
Traceback (most recent call last):
  File "C:\Program Files\Python310\lib\runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "C:\Program Files\Python310\lib\runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "mypy\semanal.py", line 4990, in accept
  File "mypy\nodes.py", line 959, in accept
  File "mypy\semanal.py", line 1078, in visit_class_def
  File "mypy\semanal.py", line 1100, in analyze_class
  File "mypy\semanal.py", line 1330, in clean_up_bases_and_infer_type_variables
AssertionError: mypy does not currently support ParamSpec use in generic classes
stdlib\contextlib.pyi:49: : note: use --pdb to drop into pdb

• Python version: 10.1
• mypy version: 0.920
• Command run: mypy test.py --custom-typeshed-dir . --tb (test.py being an empty file in the root folder of my typeshed clone).

Okay, so should be fixed come 0.930

Fixes #6625 (I think)

I can confirm that it fixes the issue in my codebase. Thanks for the fix! 🙌