Cannot call `issubclass()` against `typing.Protocol` on Python 3.12

[AlexWaygood]

Bug report

On Python 3.11, you can do this:

>>> import typing
>>> issubclass(object, typing.Protocol)
False

But on Python 3.12 (following the backport of c05c31d), this raises:

>>> import typing
>>> issubclass(object, typing.Protocol)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1797, in __subclasscheck__
    raise TypeError(
TypeError: Instance and class checks can only be used with @runtime_checkable protocols

The new behaviour doesn't make sense. That TypeError should only be triggered if a user is calling issubclass() against a subclass of typing.Protocol. typing.Protocol itself should be exempted.

Linked PRs

• gh-105237: Allow calling issubclass(X, typing.Protocol) again #105239
• [3.12] gh-105237: Allow calling issubclass(X, typing.Protocol) again (GH-105239) #105316

[AlexWaygood]

While working on fixing the bug, we discovered another bug, where issubclass(object, Protocol) can sometimes evaluate to True. Unlike the original bug, this one reproduces all the way back to Python 3.8. It's now fixed on main and 3.12, but it might be good to see if we can fix it on 3.11 as well.

[AlexWaygood]

While working on fixing the bug, we discovered another bug, where issubclass(object, Protocol) can sometimes evaluate to True. Unlike the original bug, this one reproduces all the way back to Python 3.8. It's now fixed on main and 3.12, but it might be good to see if we can fix it on 3.11 as well.

Well, but on the other hand, I'm hesitant to touch 3.11 here, given that it's an edge-case bug, and it's hard to do a proper fix that's not somewhat invasive. So, I'll close this for now. (But if somebody else disagrees and wants to have a go at fixing it on 3.11, happy to reopen.)