Failed to compile fmt on emscripten using C++20

[Floris0106]

When compiling fmt using emscripten's CMake toolchain file (obtained through the emsdk) with the following errors:

error: call to consteval function 'fmt::basic_format_string<char, const char *>::basic_format_string<FMT_COMPILE_STRING, 0>' is not a constant expression
error: call to consteval function 'fmt::basic_format_string<char, const char *>::basic_format_string<FMT_COMPILE_STRING, 0>' is not a constant expression
error: call to consteval function 'fmt::basic_format_string<char, int &>::basic_format_string<FMT_COMPILE_STRING, 0>' is not a constant expression
error: call to consteval function 'fmt::basic_format_string<char, int &, int &>::basic_format_string<FMT_COMPILE_STRING, 0>' is not a constant expression
error: call to consteval function 'fmt::basic_format_string<char, fmt::basic_string_view<char> &, const char (&)[3]>::basic_format_string<FMT_COMPILE_STRING, 0>' is not a constant expression
error: call to consteval function 'fmt::basic_format_string<char, const char (&)[7], int &>::basic_format_string<FMT_COMPILE_STRING, 0>' is not a constant expression
error: call to consteval function 'fmt::basic_format_string<char, unsigned int &>::basic_format_string<FMT_COMPILE_STRING, 0>' is not a constant expression
error: call to consteval function 'fmt::basic_format_string<char, unsigned int &>::basic_format_string<FMT_COMPILE_STRING, 0>' is not a constant expression
error: call to consteval function 'fmt::basic_format_string<char, int>::basic_format_string<FMT_COMPILE_STRING, 0>' is not a constant expression

I'm able to reproduce this issue with the latest versions of emsdk (3.1.70) and fmt (11.0.2). I used the following CMakePresets.json:

{
	"version": 1,
	"configurePresets": [
		{
			"name": "Emscripten",
			"generator": "Ninja",
			"toolchainFile": "$env{EMSDK}/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake"
		}
	]
}

Here EMSDK is an environment variable that points to the root directory of the emsdk. CMakeLists.txt:

include(FetchContent)

project(Test)

set(CMAKE_CXX_STANDARD 20)

FetchContent_Declare(fmt GIT_REPOSITORY https://github.com/fmtlib/fmt.git GIT_TAG 11.0.2 DOWNLOAD_EXTRACT_TIMESTAMP true)
FetchContent_MakeAvailable(fmt)

I'm not sure whether this is an issue with fmt or emscripten, so I've made an issue on the fmt repository as well. (see here)

[Twinklebear]

I think I hit the same and it was fixed for me by this PR on fmt: fmtlib/fmt#4187 , if you switch to pull fmt commit 22701d5f63fd9ba3ffa35fe94585b5bfcb69238b or later it should compile.

[Floris0106]

Ah perfect, now it works. Thank you!