In file included from main.cpp:19:0:
is_customly_convertible.h:7:38: error: 'void_t' was not declared in this scope
 struct is_customly_convertible<A, B, void_t<decltype(Convert<A, B>())>> :
                                      ^~~~~~
is_customly_convertible.h:7:38: note: suggested alternative:
In file included from /usr/include/c++/7/bits/move.h:54:0,
                 from /usr/include/c++/7/bits/stl_pair.h:59,
                 from /usr/include/c++/7/utility:70,
                 from /usr/include/c++/7/algorithm:60,
                 from main.cpp:1:
/usr/include/c++/7/type_traits:2505:44: note:   'std::void_t'
   template<typename...> using void_t = void;
                                            ^
In file included from main.cpp:19:0:
is_customly_convertible.h:7:70: error: template argument 3 is invalid
 struct is_customly_convertible<A, B, void_t<decltype(Convert<A, B>())>> :
                                                                      ^~
main.cpp: In function 'int main()':
main.cpp:53:5: error: static assertion failed
     static_assert(is_customly_convertible<int, int>::value);
     ^~~~~~~~~~~~~
main.cpp:55:5: error: static assertion failed
     static_assert(is_customly_convertible<int, float>::value);
     ^~~~~~~~~~~~~
main.cpp:58:5: error: static assertion failed
     static_assert(is_customly_convertible<Test, int>::value);
     ^~~~~~~~~~~~~
main.cpp:61:5: error: static assertion failed
     static_assert(is_customly_convertible<NoCopyConstructible, NoTriviallyConstructible>::value);
     ^~~~~~~~~~~~~