Incorrect pickling behavior in the c implemented classmethod descriptors

[UltimateLobster]

Bug report

Hello there, this is my first bug report so apologies if there's something I miss.

Consider the following code:

import pickle

classmethod_descriptor = dict.__dict__["fromkeys"]
unpickled_classmethod_descriptor = pickle.loads(pickle.dumps(classmethod_descriptor, protocol=5))

While it passes without raising any errors, classmethod_descriptor differs from unpickled_classmethod_descriptor. Instead of the original descriptor, the pickling process yields the function itself:

>>> classmethod_descriptor
<method 'fromkeys' of 'dict' objects>

>>> unpickled_classmethod_descriptor
<function dict.fromkeys(iterable, value=None, /)>

Manually calling the descriptor's __reduce_ex__ allows us to see the reason:

>>> classmethod_descriptor.__reduce_ex__(5)
(<function getattr>, (dict, 'fromkeys'))

This behavior differs from python implemented classmethods:

import pickle

class A:
    @classmethod
    def method(cls):
        pass

classmethod_descriptor = A.__dict__["method"]

# Any of these lines will raise "TypeError: cannot pickle 'classmethod' object"
pickle.dumps(classmethod_descriptor, protocol=5)
classmethod_descriptor.__reduce_ex__(5)

Since it fails with normal classmethods, I would expect it to fail with their c counterparts as well so their behavior would match.
Either that, or having both of them succeed during the pickling process, in which case, __reduce_ex__ should return a valid result in both cases.

The bug occurs with other c implemented class methods as well:

import pickle

from datetime import datetime

classmethod_descriptor = datetime.__dict__["now"]

# Returns False
pickle.loads(pickle.dumps(classmethod_descriptor, 5)) == classmethod_descriptor

# Returns True
pickle.loads(pickle.dumps(classmethod_descriptor, 5)) == datetime.now

Your environment

Python Versions:
I've tested it with both 3.10.5 and 3.11.04b
OS - Windows 10 Pro 64 bit

[UltimateLobster]

#96162

Opened a PR to solve the issue

[serhiy-storchaka]

Since this feature never worked correctly, and pickling of Python implemented classmethod descriptors and C and Python implemented staticmethod descriptors is not supported, and there are no common use cases for this (otherwise we would get such report years ago), and the correct implementation is not trivial if not use eval(), I thing that the best way to fix this is to disable it. There are no existing tests for this, and it looks that this code was added by mistake and was never tested.