I'm trying to add the /std:c++17 compiler flag to VS2017 with CMake. I'm using the "modern" cross-platform way so far:
set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF) # -std=c++11 instead of -std=gnu++11
set(MY_CXX_COMPILE_FEATURES cxx_generic_lambdas cxx_range_for cxx_strong_enums)
add_library(mylib INTERFACE)
target_compile_features(mylib INTERFACE ${MY_CXX_COMPILE_FEATURES})
This adds /std:c++14 in VS2017 (which might be the default anyway?).
However I'm having trouble switching this to C++17 (i.e. having it add /std:c++17). If I just add it manually, I get the not-so-nice warning because both flags are present:
1>cl : Command line warning D9025: overriding '/std:c++14' with '/std:c++17'
I've tried set(CMAKE_CXX_STANDARD 17) but it has no effect, in fact the CMake documentation mentions that CMAKE_CXX_STANDARD has no effect on VS anyway.
As for adding a C++17 feature to target_compile_features, it doesn't seem like there are any yet (even in CMake-3.9.0-rc5), and even if there were, I'm specifically only using std::optional from C++17, and there's no target_compile_features flags for library features like std::optional.
So my question is, what's the best (or least ugly) way to do this with CMake? And in a way so it'll also work for gcc and clang? I'm happy to use a very recent CMake version (3.8 or 3.9). I prefer it to be "nice" and not manually looping through CXX_COMPILE_FLAGS and removing the string "/std:c++14" or some hack like that.
(Edit: It can also be the VS/std:c++latest switch - whichever is possible. Both work for the purpose.)