Abort from `GenericAlias.__sizeof__`: `ob->ob_type != &PyLong_Type`

[devdanzin]

Crash report

What happened?

It's possible to abort the interpreter in 3.12 by calling __sizeof__ of a GenericAlias instance:

from types import GenericAlias

g = GenericAlias(BaseException, Exception)
g.__sizeof__(1)

Abort message:

python: ./Include/object.h:231: Py_SIZE: Assertion `ob->ob_type != &PyLong_Type' failed.
Aborted

Found using fusil by @vstinner.

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.12.8+ (heads/3.12:8e3c2d2, Dec 4 2024, 10:58:33) [GCC 11.4.0]

[vstinner]

I created #127605 backport to fix the issue.

[vstinner]

Fixed by commit 0964f9f. Thanks for your bug report.