[BUG]: token pasting of ',' and __VA_ARGS__ is a GNU extension / pedantic warnings

[N-Wouda]

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What version (or hash if on master) of pybind11 are you using?

2.13.3

Problem description

The most recent release uses , ##__VA_ARGS__ in common.h. This is not technically in the standard (see e.g. here), but most compilers support it. Clang, however, complains about its use when setting -Wpedantic. I can work around this by either not passing -Wpedantic, or passing -Wno-gnu-zero-variadic-macro-arguments, but I'd rather not do that if it can at all be avoided.

Consider the attached example, test.cpp. Compiling it with

clang -Wall -Wextra -Wpedantic -Werror -shared -std=c++20 -fPIC $(python -m pybind11 --includes) test.cpp -o test.so

produces the following:

In file included from test.cpp:1:
In file included from .venv/lib/python3.11/site-packages/pybind11/include/pybind11/pybind11.h:13:
In file included from .venv/lib/python3.11/site-packages/pybind11/include/pybind11/detail/class.h:12:
In file included from .venv/lib/python3.11/site-packages/pybind11/include/pybind11/attr.h:13:
.venv/lib/python3.11/site-packages/pybind11/include/pybind11/detail/common.h:493:57: error: token pasting of ',' and __VA_ARGS__ is a GNU extension [-Werror,-Wgnu-zero-variadic-macro-arguments]
            &PYBIND11_CONCAT(pybind11_module_def_, name),
  \
                                                        ^
1 error generated.

It concerns the following code snippet:

pybind11/include/pybind11/detail/common.h

Lines 493 to 494 in fc97cc4

	&PYBIND11_CONCAT(pybind11_module_def_, name), \
	##__VA_ARGS__); \

I am not sure if pybind needs to consider this a bug or even an issue. But the previous versions of pybind (2.12 and below) worked without a hitch when setting the -Wpedantic option, so I figured I'd let you know about this here.

Reproducible example code

#include <pybind11/pybind11.h>

int test()
{
    return 42;
}

PYBIND11_MODULE(module, m)
{
    m.def("test", &test);
}

Is this a regression? Put the last known working version here if it is.

2.12.0

[henryiii]

PYBIND11_MODULE(module, m, py::mod_gil_not_used()) would fix it, I think?

We probably should push and pop this warning, then.

[henryiii]

I don't think we can fully hide the warning inside a macro. There are actually three cases, the others have been in the code base forever but are less common. In #5322 I've managed to get it working with the flag for C++20 mode. Before C++20, GCC's warning isn't suppressible, since it's not categorized (it's just -Werror), and GCC doesn't seem to be reliable about allowing it to be suppressed as -Wpedantic. And Clang can produce another error code here before C++20, which probably isn't in older compilers, so I think supporting C++20 for pedantic is probably fine.

[N-Wouda]

Thanks @henryiii! In my use case we compile everything in C++20 mode, so this would solve my particular issue. As you mention it is difficult to backport this to earlier C++ modes, I think supporting 20 and up should suffice for most (new) projects. In any case I'm a big fan of supporting -Wpedantic builds 👍.