Compilation fails with C++20 on emscripten's toolchain

[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 emscripten repository as well. (see here)

[tchaikov]

@Floris0106 i think it's fixed by #4187. but if you want to build with fmt 11.0.2, the patch might not apply cleanly. i'd suggest use the patch at https://src.fedoraproject.org/rpms/fmt/c/8da7a020a557972c246e4b8c927c3108c492e83c?branch=rawhide which contains a backport to 11.0.2.

[Floris0106]

I don't know if I'm doing something wrong, but the fedora repo only gives me some source and patch files, both using FetchContent and when manually cloning. Anyway, I'll close this since it's fixed in 22701d5 (which is what I'm using now).

[tchaikov]

@Floris0106 for applying a patch when using FetchContent_Declare() you might want to use PATCH_COMMAND like

FetchContent_Declare(fmt
  GIT_REPOSITORY https://github.com/fmtlib/fmt.git
  GIT_TAG 11.0.2
  PATCH_COMMAND git apply --ignore-whitespace "${CMAKE_CURRENT_LIST_DIR}/path-to-patch.patch"
  DOWNLOAD_EXTRACT_TIMESTAMP true)