Language request: C++14

Hello,

I have an issue with current Mean-max puzzle with C++ language.

In my code I am using the following construct:
template < typename…>
using void_t = void;

this compile perfectly with gcc 7.2 with the following flag -std=c++14 -O0 -g
But it doesn’t compile with -std=c++11.

When submitting on Codingame I encouter errors, like if it was compiled with -std=c++11 (or no flag at all)
I don’t understand why because according the codingame FAQ: g++ 7.2.0 mode C++14 should be supported.

What I am missing ?

EDIT: update code that was rendered incorrectly