Casting error on Clang 15 (and 17) and GCC 13 about spdlog

[tt4g]

spdlog does not recognize that the compiler supports std::format_string, so it uses std::string_view as the format argument.

spdlog/include/spdlog/common.h

Lines 145 to 156 in 73e2e02

	#ifdef SPDLOG_USE_STD_FORMAT
	namespace fmt_lib = std;
	using string_view_t = std::string_view;
	using memory_buf_t = std::string;
	template <typename... Args>
	#if __cpp_lib_format >= 202207L
	using format_string_t = std::format_string<Args...>;
	#else
	using format_string_t = std::string_view;
	#endif

C++23 is required because std::format_string is a C++23 feature.
I think you have not enabled it with compiler flags.

[sfulham]

Thanks @tt4g, it seems as if, even with c++23 set, __cpp_lib_format is set to 202106 with libstdc++13.1 and undefined with libc++-17. I'm on PopOS 22.04 so some packages may be a little behind, but these are the latest packages I can seem to find.

[sfulham]

It seems if I manually define __cpp_lib_format to 202207L with libc++-17 it works fine, however this is really not a permanent solution.

[tt4g]

__cpp_lib_format >= 202207L should be a prerequisite for std::format_string support.

See __cpp_lib_format section in Feature testing - cppreference.com and P2508R1.

The fact that it is 202106 means that the compiler does not support std::format_string, so using std::format_string in Log::PrintMessage is a mistake.

[tt4g]

It seems if I manually define __cpp_lib_format to 202207L with libc++-17 it works fine, however this is really not a permanent solution.

If so, it may be a compiler bug.
Please report it to the compiler maintainer.

[sfulham]

It seems if I manually define __cpp_lib_format to 202207L with libc++-17 it works fine, however this is really not a permanent solution.

If so, it may be a compiler bug. Please report it to the compiler maintainer.

After some more digging, it seems it isn't a mistake/it is known to be undefined, https://libcxx.llvm.org/FeatureTestMacroTable.html

[tt4g]

spdlog cannot use the feature that the compiler does not support.
You should not use std::format_string.