Fix compiler detection with clang-cl

[tobiasleibner]

Description

I am getting compiler warnings using clang-cl on Windows. These warnings are suppressed in pybind11 using PYBIND11_WARNING_DISABLE_CLANG which is defined as

#ifdef PYBIND11_COMPILER_CLANG
#    define PYBIND11_WARNING_DISABLE_CLANG(name) PYBIND11_PRAGMA(clang diagnostic ignored name)
#else
#    define PYBIND11_WARNING_DISABLE_CLANG(name)
#endif

However, for clang-cl, PYBIND11_COMPILER_CLANG is undefined because clang-cl also defines _MSC_VER and thus the compiler is detected as msvc.

Suggested changelog entry:

• Fix compiler detection in pybind11/detail/pybind11_namespace_macros.h for clang-cl on Windows, to fix warning suppression macros.

[rwgk]

From the PR description:

but that macro is defined to nothing

Is that correct? I'm thinking it must be using the MSVC definitions? Could you please review the PR description for correctness and also make the suggested changelog more specific (mention warning suppression pragmas)?

[rwgk]

Could you please try to move this block last (below __GNUC__), with a small but carefully written comment to explain why it should stay there? (Your favorite LLM can probably help doing this quickly.)

[tobiasleibner]

Good idea, done.

[tobiasleibner]

Is that correct? I'm thinking it must be using the MSVC definitions? Could you please review the PR description for correctness and also make the suggested changelog more specific (mention warning suppression pragmas)?

Yes, the definition is

#ifdef PYBIND11_COMPILER_CLANG
#    define PYBIND11_WARNING_DISABLE_CLANG(name) PYBIND11_PRAGMA(clang diagnostic ignored name)
#else
#    define PYBIND11_WARNING_DISABLE_CLANG(name)
#endif

I updated the PR description to include more details.

[rwgk]

Thanks!