PYBIND11_OBJECT_CVT should use namespace for error_already_set()

[oleksandr-pavlyk]

This change makes the macro usable outside of pybind11 namespace.

Description

Change to PYBIND11_OBJECT_CVT macro to fully qualify error_already_set used in constructors. This change allows macro to be used outside of pybind11 namespace.

Suggested changelog entry:

``PYBIND11_OBJECT_CVT`` and ``PYBIND11_OBJECT_CVT_DEFAULT`` macro can be used to define classes in namespaces other than pybind11.

[rwgk]

How did you discover this?
Is there a chance you could add a test based on the situation you had? Otherwise it's likely that this will get broken again in refactoring work in the future.
Recently we had a similar PR, with added test: #3758

[oleksandr-pavlyk]

I discovered it by building a toy pybind11 extensions a.cpp in https://github.com/oleksandr-pavlyk/cython_using_number_magic_methods.git

To test the change I can try building a simple class outside of pybind11 namespace which wraps PyFloatObject inside.
Let me try to whip something up and add it to this PR.

[oleksandr-pavlyk]

Test has been added in test/test_pytypes.cpp and companion .py file. Please review again.

[rwgk]

Awesome, thanks!
Just waiting for the CI to finish.

[rwgk]

valgrind is detecting a leak and clang-tidy has two complaints:

/__w/pybind11/pybind11/tests/test_pytypes.cpp:25:7: error: do not use 'else' after 'return' [readability-else-after-return,-warnings-as-errors]
    } else {
      ^~~~~~
/__w/pybind11/pybind11/tests/test_pytypes.cpp:577:48: error: the parameter 'x' is copied for each invocation but only used as a const reference; consider making it a const reference [performance-unnecessary-value-param,-warnings-as-errors]
    m.def("square_float_", [](external::float_ x) -> double {
                                               ^

I missed the leak when I looked at the test code before. Looking again.

[oleksandr-pavlyk]

valgrind is detecting a leak and clang-tidy has two complaints:

/__w/pybind11/pybind11/tests/test_pytypes.cpp:25:7: error: do not use 'else' after 'return' [readability-else-after-return,-warnings-as-errors]
    } else {
      ^~~~~~
/__w/pybind11/pybind11/tests/test_pytypes.cpp:577:48: error: the parameter 'x' is copied for each invocation but only used as a const reference; consider making it a const reference [performance-unnecessary-value-param,-warnings-as-errors]
    m.def("square_float_", [](external::float_ x) -> double {
                                               ^

I missed the leak when I looked at the test code before. Looking again.

I am about to push a change to fix this.

[oleksandr-pavlyk]

@rwgk Thanks for your help. The CI is now green.

[rwgk]

Thanks @oleksandr-pavlyk!