...

cxx/mcc_traits.h

@@ -124,15 +124,26 @@ template <typename F>
 struct mcc_func_traits<F&&> : mcc_func_traits<F> {
 };
 
+// type of the returned value
 template <typename T>
 using mcc_retval_t = typename mcc_func_traits<T>::ret_t;
 
+// type of the first argument of callable
 template <typename T>
 using mcc_func_arg1_t = typename mcc_func_traits<T>::arg1_t;
 
+// type of the N-th argument of callable
+// NOTE: starts from 1 not from 0!!!
 template <typename T, size_t N = 1>
-using mcc_func_argN_t = std::
-    conditional_t<N >= mcc_func_traits<T>::arity, std::tuple_element_t<N, typename mcc_func_traits<T>::args_t>, void>;
+using mcc_func_argN_t = std::conditional_t<N >= mcc_func_traits<T>::arity,
+                                           std::tuple_element_t<N - 1, typename mcc_func_traits<T>::args_t>,
+                                           void>;
+
+
+// non-const lvalue reference, constructible from CtorArgTs (an output argument of function)
+template <typename T, typename... CtorArgTs>
+concept mcc_output_arg_c = !std::is_const_v<std::remove_reference_t<T>> && std::is_lvalue_reference_v<T> &&
+                           std::constructible_from<std::remove_reference_t<T>, CtorArgTs...>;
 
 namespace details
 {