[BUG]: Potential null pointer dereference after update to pybind11 3.0

[yamatveev]

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What version (or hash if on master) of pybind11 are you using?

3.0.0

Problem description

Hi, after update from 2.13.6 to 3.0.0 for python >=3.11 I started to get :

      inlined from 'void pybind11::cpp_function::initialize_generic(unique_function_record&&, const char*, const std::type_info* const*, pybind11::size_t)' at /home/matveyev/miniforge3/envs/tangodev_13/lib/python3.13/site-packages/pybind11/include/pybind11/pybind11.h:605:30:
  /home/matveyev/miniforge3/envs/tangodev_13/include/python3.13/cpython/methodobject.h:48:16: error: potential null pointer dereference [-Werror=null-dereference]
     48 |         return _Py_NULL;
        |                ^~~~~~~~
  cc1plus: all warnings being treated as errors

However, somehow it is not 100% obvious for me, what was changed in 3.0 wrt 2.13, that this appeared. Any hints?

Reproducible example code

Is this a regression? Put the last known working version here if it is.

2.13.6

[yamatveev]

The problem is caused by #5580 , but still do not understand how