[CMake] target_compile_features() uses incorrect C++ version

[Deniz Bahadir]

Am 05.12.18 um 13:40 schrieb Kim Walisch:
> Hi,
> 
> I have realized that my C++ primesieve project 
> (https://github.com/kimwalisch/primesieve) is
> compiled using -std=gnu++11 with GCC 7.3 and CMake 3.10 (on Ubuntu 18.10 
> x64) even
> though the default C++ version used by GCC 7.3 is C++14 as checked below:
> 
> $ g++ -dM -E -x c++  /dev/null | grep -F __cplusplus
> #define __cplusplus 201402L
> 
> Note that I have not set CXX_STANDARD manually in my CMakeLists.txt so I 
> was expecting
> that my project would be compiled either with -std=gnu++14 or without 
> any C++ version flag.
> I spent some time investigating where the -std=gnu++11 flag comes from 
> and I found that it is
> related to the use of target_compile_features():
> 
> target_compile_features(primesieve PRIVATE cxx_auto_type)
> 
> This code tells the compiler that my program uses features from C++11 
> and that the compiler
> should enable C++11 if the default C++ version of the compiler is e.g. 
> C++98. I would
> however expect that if the default C++ version of the compiler is > 
> C++11 CMake would not
> add -std=gnu++11 to the compiler flags.
> 
> For me this is a CMake bug.
> 
> Thanks,
> Kim Walisch
> 

Hi Kim,

this does indeed sound like some unwanted behavior.

I recommend you open an issue in CMake's issue-tracker 
(https://gitlab.kitware.com/cmake/cmake/issues) and copy your 
email-content there.

Best regards,
Deniz Bahadir