const error_category& future_category() noexcept;
Returns: A reference to an object of a type derived from class error_category.
The object's default_error_condition and equivalent virtual functions shall behave as specified for the class error_category. The object's name virtual function shall return a pointer to the string "future".
error_code make_error_code(future_errc e) noexcept;
Returns: error_code(static_cast<int>(e), future_category()).
error_condition make_error_condition(future_errc e) noexcept;
Returns: error_condition(static_cast<int>(e), future_category()).