fix: enable bind_map with using declarations.

[AntoinePrv]

Description

Enable using py::bind_map with map-like things that privately inherit from map + using declarations.

Suggested changelog entry:

- Fix ``bind_map`` with ``using`` declarations.

[rwgk]

Could you please add a test that does not work without this PR? — That's the only way to ensure that there are no regressions long-term.

See tests/test_stl_binders.cpp and maybe tests/test_stl_binders.py

[rwgk]

LGTM but could you please also add minimal tests in test_stl_bind.py? Just enough to be sure the new py::bind_vector and py::bind_map are not accidentally lost in the future.

[AntoinePrv]

LGTM but could you please also add minimal tests in test_stl_bind.py? Just enough to be sure the new py::bind_vector and py::bind_map are not accidentally lost in the future.

IMHO this is not necessary. This was purely a compilation fix.

[rwgk]

LGTM but could you please also add minimal tests in test_stl_bind.py? Just enough to be sure the new py::bind_vector and py::bind_map are not accidentally lost in the future.

IMHO this is not necessary. This was purely a compilation fix.

That's what I understood.

But there is a chance that the two new lines are accidentally removed when refactoring.

Adding minimal tests (maybe 2 x 2 lines) is very easy and makes it much less likely that something gets lost accidentally.

[AntoinePrv]

I have added a Python test. Thanks for taking the time.

[rwgk]

Thanks, looks great!

[henryiii]

This seems to be the only obvious fix. Lambdas can be more expensive to compile, but I think that's with capture, which isn't used for these. So I think it's fine.