Assuming we want to go with #[pyclass(generic)] as the syntax, I think this is roughly the best way to insert that method.

As for the error: what this builder does is that it a) emits an extern "C" fn that wraps a rust function, and b) emits code that gives Python a pointer to the function and its name. It does not emit the rust function; you have to add that somewhere. Probably something like this:

This should also prevent the user from both writing #[pyclass(generic)] and creating a method named __class_getitem__ (please add a test for this)

I added tests and updated the documentation. I am confident it all works. For compilation failure, I tested users redefining __class_geitem__ and blocked enums for the time being.

I am still not sure how to block #[pyclass(generic)] for Python 3.7 and Python 3.8. I don't know if we should do it, but currently specifying it will cause compilation failures anyway as #4917 is gated for Python 3.9+