Mark namespace scope constexpr variable 'buffer_size' inline.

[kamrann]

This tiny change works around possible clang bug which currently prevents compilation on clang with modules and FMT_ATTACH_TO_GLOBAL_MODULE. From my understanding, std library implementations changed such constexpr namespace scope variables to be inline after C++17, so I think this is perhaps a good change regardless of whether clang is wrong here or not (from my limited understanding, I think it protects against possible ODR violations and has no downside).

With both this and #4083, I can successfully build fmt with modules + FMT_ATTACH_TO_GLOBAL_MODULE on Windows+MSVC, Windows+Clang and Linux+Clang.

[vitaut]

Similarly to the other case, could you post the full error message?

[kamrann]

For reference.

Command line: C:/LLVM/build-18x/Release/bin/clang++ -DFMT_ATTACH_TO_GLOBAL_MODULE -DFMT_MODULE -IC:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt/include -IC:\Coding\Build2\third-party-packaging\build2-packaging\build\clang-release\fmt -IC:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt -std=c++2b -D__cpp_modules=201907L -Wno-unqualified-std-cast-call -fdiagnostics-color -fansi-escape-codes -finput-charset=UTF-8 -Xclang -fexternc-nounwind -D_MT -D_DLL -Xclang -fmodules-embed-all-files -fmodule-output=..\build\clang-release\fmt\fmt\src\fmt.lib.pcm -o ..\build\clang-release\fmt\fmt\src\fmt.lib.pcm.obj -c -x c++-module C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt\src\fmt.cc

Error output:

In file included from C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt\src\fmt.cc:113:
C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt/include\fmt/os.h:386:16: error: declaration of 'buffer_size' with internal linkage cannot be exported
  386 | constexpr auto buffer_size = detail::buffer_size();
      |                ^
C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt/include\fmt/os.h:77:1: note: export block begins here
   77 | FMT_BEGIN_EXPORT
      | ^
C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt\src\fmt.cc:89:26: note: expanded from macro 'FMT_BEGIN_EXPORT'
   89 | #define FMT_BEGIN_EXPORT export {
      |                          ^

[vitaut]

Thanks for the fix!