Add FMT_STRING for format_to() call with clang-cl

[mogemimi]

I get the following error when compiling os.cc on clang-cl 15.

1>..\fmt\src\os.cc(172,52): error : call to consteval function 'fmt::basic_format_string<char, const char *&, fmt::detail::utf16_to_utf8 &>::basic_format_string<char[7], 0>' is not a constant expression
1>..\fmt\include\fmt/core.h(3165,62): message : non-constexpr constructor 'basic_string_view' cannot be used in a constant expression
1>..\fmt\src\os.cc(172,52): message : in call to 'basic_format_string("{}: {}")'
1>..\fmt\include\fmt/core.h(451,3): message : declared here

This pull request adds a FMT_STRING wrapper to resolve a format-string type and fixes the error.

[vitaut]

FMT_STRING shouldn't be needed here. Please provide a complete repro, ideally on godbolt.

[mogemimi]

@vitaut
Thank you for replying. When I compiled os.cc with /std:c++20 and -fmsc-version=1913 on clang-cl 15, the above error occurred. I had overlooked that I had specified -fmsc-version= option.

The reproduction procedure is as follows

1. Install LLVM 15 on Windows. Or install clang-cl 15 from Visual Studio 2022 installer.

   # "/c/Program Files/LLVM/bin/clang-cl.exe" --version
   
   clang version 15.0.7
   Target: x86_64-pc-windows-msvc
   Thread model: posix
   InstalledDir: C:\Program Files\LLVM\bin

   # "/c/Program Files/Microsoft Visual Studio/2022/Community/VC/Tools/Llvm/bin/clang-cl.exe" --version
   
   clang version 15.0.1
   Target: i686-pc-windows-msvc
   Thread model: posix
   InstalledDir: C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\Llvm\bin

2. Go to the fmt directory.

   # cd path/to/fmt
   # ls src/os.cc

3. Run the following command to compile src/os.cc using clang-cl:

   # "/c/Program Files/LLVM/bin/clang-cl.exe" -Xclang -Iinclude /std:c++20 -fmsc-version=1913 /EHsc /diagnostics:column -Xclang -fcxx-exceptions src/os.cc
   
   src/os.cc(172,52): error: call to consteval function 'fmt::basic_format_string<char, const char *&, fmt::basic_string_view<char>>::basic_format_string<char[7], 0>' is not a constant expression
   include\fmt/core.h(2758,62): note: non-constexpr constructor 'basic_string_view' cannot be used in a constant expression
   src/os.cc(172,52): note: in call to 'basic_format_string("{}: {}")'
   include\fmt/core.h(421,3): note: declared here

In addition, when I specified /std:c++14 or /std:c++17 instead of /std:c++20, the error did not occur. Also, specifying 1914 or higher for -fmsc-version= like -fmsc-version=1914 did not cause an error.

The reason seems to be that FMT_CONSTEXPR_CHAR_TRAITS is enabled when FMT_MSC_VERSION is 1914 or higher.

fmt/include/fmt/core.h

Lines 124 to 126 in 02cae7e

	#elif FMT_MSC_VERSION >= 1914 && FMT_CPLUSPLUS >= 201703L
	# define FMT_CONSTEXPR_CHAR_TRAITS constexpr
	#endif

[vitaut]

I think it's OK as a workaround considering that we use FMT_STRING in similar cases elsewhere. Thanks.