If we run GCC 7.1, we can as well use -std=gnu++17

It is not an option currently. There is no -std=gnu++17 option in GCC 7.1 https://gcc.gnu.org/onlinedocs/gcc-7.1.0/gcc/C-Dialect-Options.html#C-Dialect-Options . -std=gnu++1z means, that its implementation is not fully complete.

Edited by author 01.03.2018 10:38

That document was not updated, "std=c++17" is supported in GCC since 2014, see <https://github.com/gcc-mirror/gcc/commit/4b53bc0f989bb7e98b21494a7a92302f324594d0>

According to <https://gcc.gnu.org/projects/cxx-status.html#cxx17> all the C++17 features are implemented in GCC 7.

I have no G++ 7.1 to check. But G++ 7.2 said you are correct.