Bad error message when using incorrect signature for some dunder methods

Bug Description

When implementing __iadd__, I assumed that I'd need to return a reference to self since that's generally the behavior expected in python. Following that logic, I wrote it with the signature:

fn __iadd__<'a>(self_: PyRefMut<'a, Self>, other: &Self) -> PyRefMut<'a, Self>.

This lead to a compiler error message about PyRefMut not implementing the correct conversion trait, which lead me down a bit of a rabbit hole until I found the correct section in the readme.

Ideally a dunder method with an incorrect signature could be detected, and the error message note the expected signature or just point users to the relevant section of the readme.

Steps to Reproduce

Define a pyclass with
fn __iadd__<'a>(self_: PyRefMut<'a, Self>, other: &Self) -> PyRefMut<'a, Self>{...} in the impl block, observe the error message

Backtrace

No response

Your operating system and version

Windows 11

Your Python version (python --version)

Python 3.12.4

Your Rust version (rustc --version)

rustc 1.80.1 (3f5fd8dd4 2024-08-06)

Your PyO3 version

0.22.1

How did you install python? Did you use a virtualenv?

Basic installer for python, UV 0.3 used for virtualenv

Additional Info

No response

Start by reproducing the reported iadd signature in a PyO3 pyclass impl and compare the compiler diagnostic with the relevant dunder-method section of the README. Trace the pyclass or dunder-method validation entry point that handles the signature, then verify that an incorrect signature produces an actionable expected-signature or documentation diagnostic.