CMAKE_CXX_STANDARD should be c++20 if SPDLOG_USE_STD_FORMAT is set

[mdavis199]

By default, spdlog sets CMAKE_CXX_STANDARD 11. However, if the option SPDLOG_USE_STD_FORMAT is ON, the required standard should be 17 or higher, because std::string_view is required.

This can cause problems when using package managers such as vcpkg or conan. To enable SPDLOG_USE_STD_FORMAT, package managers need to patch spdlog's CMakelist.txt.

I suggest that spdlog's CMakeLists.txt should specify the CMAKE_CXX_STANDARD through target_compile_features based on the value of SPDLOG_USE_STD_FORMAT.

[tt4g]

Right. PR is welcome.

Workaround. if we specify cmake -DCMAKE_CXX_STANDARD=20 (other options...) at build time, the problem should not appear.

[cookcocck]

@mdavis199
What if we specify CMAKE_CXX_STANDARD by target_compile_features, but also want to pass the parameter via cmake -DCMAKE_CXX_STANDARD?

[cookcocck]

@tt4g Could you please check my PR?
#2402

[sylveon]

Seems like this can be closed