variadic: rework get to infer references in return types

variadic.hpp

@@ -41,18 +41,38 @@ namespace util::variadic {
 
 
     ///////////////////////////////////////////////////////////////////////////
+    namespace detail {
+        // using the strategy of discarding assignment from:
+        //     https://attugit.github.io/2015/02/Accessing-nth-element-of-parameter-pack/
+        struct discard {
+            template <typename ArgsT>
+            constexpr discard (ArgsT&&...) { ; }
+        };
+
+        template <size_t>
+        using discard_n = discard;
+
+        template <std::size_t N, typename = std::make_index_sequence<N>>
+        struct getter;
+
+        template <std::size_t N, std::size_t... ignore>
+        struct getter<N, std::index_sequence<ignore...>> {
+            template <typename ValueT, typename ...TailT>
+            decltype(auto)
+            operator() (discard_n<ignore>..., ValueT &&val, TailT&&...) const
+            {
+                return std::forward<ValueT> (val);
+            }
+        };
+    }
+
     /// Returns the argument at index `IndexV', as if we called:
     ///     std::get<N> (std::make_tuple (...))
-    template <std::size_t IndexV, typename HeadT, typename ...TailT>
+    template <size_t IndexV, typename ...ArgsT>
-    auto
+    decltype(auto)
-    get (HeadT &&head, TailT &&...tail) noexcept ((std::is_nothrow_move_constructible_v<TailT> && ...))
+    get (ArgsT&& ...args)
     {
-        static_assert (IndexV < sizeof... (TailT) + 1, "Index is out of bounds");
+        return detail::getter<IndexV> {}(std::forward<ArgsT> (args)...);
-
-        if constexpr (IndexV == 0)
-            return std::forward<HeadT> (head);
-        else
-            return get<IndexV-1> (std::forward<TailT> (tail)...);
     }