dataclass(slots=True) fails if method has an empty `__class__` cell

[JelleZijlstra]

Bug report

Bug description:

On Python 3.14, this code:

def maker():
 if False:
 __class__ = 42

 def method(self):
 return __class__
 return method

from dataclasses import dataclass

@dataclass(slots=True)
class X:
 a: int

 meth = maker()

Fails with ValueError: Cell is empty, because some dataclasses machinery for updating the __class__ cell doesn't reckon with the possibility that the cell may be empty.

CPython versions tested on:

3.14, CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-148947: dataclasses: fix error on empty __class__ cell  #148948
• [3.14] gh-148947: dataclasses: fix error on empty __class__ cell (GH-148948) #148995