compile error on Windows with external fmtlib and MSVC 17.3.6

[YunchengLiu]

Thanks a lot for this excellent library, helped me a lot.
But I encountered a sudden problem.
On Windows platform, after enabling wchar_t and external fmtlib support in spdlog, when compiling with C++20 option in MSVC 17.3.6, fmt throws a compilation error.

spdlog ( 1.10.0#1) and fmtlib (9.1.0) are all installed via vcpkg(x64-windows) with lasted port comfigurations.

Message as follows:

Error C7595: "fmt:: v9:: basic_format_string<wchar_t, const tm&>:: basic_format_string": The call to an instant function is not a constant expression.

The wrong location is:

spdlog/include/spdlog/sinks/daily_file_sink.h

Line 81 in d011332

return fmt::format(fmt_filename, now_tm);

I saw the comments here, but I still tried to modify it to fmt::runtime , it compiles successfully at this time, it seems that the comments here are a bit outdated? Has the new version of MSVC resolved the contradiction between wchar_t and fmt?

I tried to reproduce the above problem externally with the following minimal code. On my local PC, the following code using MSVC 17.3.6 will throw the same error (not modified to fmt::runtime ).

#define SPDLOG_WCHAR_FILENAMES
#define SPDLOG_FMT_EXTERNAL
#include <fmt/format.h>
#include <spdlog/spdlog.h>
#include <spdlog/sinks/daily_file_sink.h>

int main(){

    auto logger = spdlog::daily_logger_mt("daily_logger", L"logs/daily.txt", 2, 30);

    logger->info("{} world", "hello");

}

Since I don't know about other platforms and the specific requirements for compiler versions, I would like to ask if it is possible to modify this to fmt::runtime to solve the problem?

[tt4g]

fmt recently added support for wcart_t * compilation time checks (fmtlib/fmt#3107 (comment)).

The fmt bundled with spdlog is version 8.x, so it does not take into account the compile check feature of the latest version.

Can you add the SPDLOG_FMT_RUNTIME macro to avoid the problem?
Also, if it works, could you please create a PR?

spdlog/include/spdlog/sinks/daily_file_sink.h

Lines 80 to 84 in d011332

	# if defined(_MSC_VER) && defined(SPDLOG_WCHAR_FILENAMES) // for some reason msvc doesn't allow fmt::runtime(..) with wchar here
	return fmt::format(fmt_filename, now_tm);
	# else
	return fmt::format(SPDLOG_FMT_RUNTIME(fmt_filename), now_tm);
	# endif