22 General utilities library [utilities]
22.10.6 Class template reference_wrapper [refwrap]
template<class... ArgTypes>
  constexpr invoke_result_t<T&, ArgTypes...>
    operator()(ArgTypes&&... args) const noexcept(is_nothrow_invocable_v<T&, ArgTypes...>);
Mandates: 
T is a complete type
. Returns: 
INVOKE(get(), std::forward<ArgTypes>(args)...).   (
[func.require])