c++ - How to enable /std:c++17 in VS2017 with CMake

Question

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.)

Answer 1

Turning my comment into an answer

1. The CMake team is working on it for VS2017 (as for July 2017, for upcoming CMake version 3.10):

   CMake: MSVC standard version switches

   Those flags seem to be rather new switches (as related to the date of this question):

   VS 2017 15.3 preview now supports /std:c++17

   So for Visual Studio you have to "manually" replace or append the compiler switches until CMake officially does support it.

   Here is a code snippet that I've tested for std:c++latest (which is already supported e.g. in my CMake 3.8.0 version):

   if (MSVC_VERSION GREATER_EQUAL "1900")
       include(CheckCXXCompilerFlag)
       CHECK_CXX_COMPILER_FLAG("/std:c++latest" _cpp_latest_flag_supported)
       if (_cpp_latest_flag_supported)
           add_compile_options("/std:c++latest")
       endif()
   endif()
   

2. For CLang and GNU the support was merged into the main source code branch begin of 2017 and is part of CMake version 3.8 and above:

   CMake: Features: Add support for C++ 17 language standard