Ok, I think I figured it out now.

Essentially, Cython's functions type, despite starting with the same struct layout as PyCFunction, must not visibly inherit from PyCFunction. Consequently, inspect.isbuiltin() returns False, as does inspect.isfunction() - sadly.

With Yury's changes that are currently committed, this triggers the fallback that checks for the function-like signature, which in turn makes Signature.from_function() properly analyse the signature, including default arguments, annotations, etc.

So, nothing left to change on CPython side for this ticket. And thanks again for the help. Closing as fixed.