...

common/adc_traits.h

@@ -55,6 +55,7 @@ struct adc_func_traits_helper_t<R> {
     using ret_t = R;
     using args_t = std::tuple<>;
     using arg1_t = void;
+    static constexpr size_t arity = 0;
 };
 
 template <typename R, typename Arg, typename... Args>
@@ -62,6 +63,7 @@ struct adc_func_traits_helper_t<R, Arg, Args...> {
     using ret_t = R;
     using args_t = std::tuple<Arg, Args...>;
     using arg1_t = Arg;
+    static constexpr size_t arity = sizeof...(Args) + 1;
 };
 
 
@@ -109,7 +111,7 @@ using adc_func_arg1_t = typename adc_func_traits<T>::arg1_t;
 template <typename T>
 using adc_deduced_type =
     std::conditional_t<std::is_lvalue_reference_v<T> && !std::is_const_v<std::remove_reference_t<T>>,
-                       T,
+                       T&,
                        std::remove_cvref_t<T>>;