In file included from main.cpp:19:0:
is_customly_convertible.h:25:68: error: 't' was not declared in this scope
 static constexpr decltype(std::declval<Convert<Q, Z>>().operator()(t),
                                                                    ^
is_customly_convertible.h:25:68: error: 't' was not declared in this scope
main.cpp: In function 'int main()':
main.cpp:58:5: error: static assertion failed
     static_assert(is_customly_convertible<Test, int>::value);
     ^~~~~~~~~~~~~