Wrap private module fragment content within conditional extern "C++"

[kamrann]

This change fixes some errors generated by clang when building with modules, where it complained that with FMT_ATTACH_TO_GLOBAL_MODULE enabled the private module fragment definitions did not match their declarations.

MSVC did not reject the original code, however I've verified that MSVC is still happy after this change.

[vitaut]

Thanks for the PR. LGTM but could you post the full error (including context) for future reference?

[kamrann]

Sure. When compiling on clang 18 with -DFMT_MODULE -DFMT_ATTACH_TO_GLOBAL_MODULE, the following is generated (along with identical errors for many other symbols):

In file included from C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt\src\fmt.cc:131:
In file included from C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt\src\format.cc:8:
C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt/include\fmt/format-inl.h:32:15: error: declaration of 'assert_fail' in module fmt follows declaration in the global module
   32 | FMT_FUNC void assert_fail(const char* file, int line, const char* message) {
      |               ^
C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt/include\fmt\base.h:394:27: note: previous declaration is here
  394 | FMT_NORETURN FMT_API void assert_fail(const char* file, int line,
      |                           ^
In file included from C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt\src\fmt.cc:131:
In file included from C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt\src\format.cc:8:
C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt/include\fmt/format-inl.h:41:15: error: declaration of 'format_error_code' in module fmt follows declaration in the global module
   41 | FMT_FUNC void format_error_code(detail::buffer<char>& out, int error_code,
      |               ^
C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt/include\fmt\format.h:3863:14: note: previous declaration is here
 3863 | FMT_API void format_error_code(buffer<char>& out, int error_code,
      |              ^
In file included from C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt\src\fmt.cc:131:
In file included from C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt\src\format.cc:8:
C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt/include\fmt/format-inl.h:64:15: error: declaration of 'report_error' in module fmt follows declaration in the global module
   64 | FMT_FUNC void report_error(format_func func, int error_code,
      |               ^
C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt/include\fmt\format.h:3867:14: note: previous declaration is here
 3867 | FMT_API void report_error(format_func func, int error_code,
      |              ^

Full command line for reference: 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

[vitaut]

Merged, thanks!