[bugfix] Use decltype(auto) for return value

If the return value can be a temporary or a reference
one has to use decltype(auto). A pure auto would create
a temporary copy of a returned reference.

dune/solvers/common/algorithm.hh

@@ -91,19 +91,19 @@ namespace Imp {
 
   template<class Container, class Index,
     std::enable_if_t<IsTuple<std::decay_t<Container>>::value, int> = 0>
-  constexpr auto elementAt(Container&& c, Index&&, PriorityTag<2>)
+  constexpr decltype(auto) elementAt(Container&& c, Index&&, PriorityTag<2>)
   {
     return std::get<Index::value>(c);
   }
 
   template<class T, T... t, class Index>
-  constexpr auto elementAt(std::integer_sequence<T, t...> c, Index&&, PriorityTag<1>)
+  constexpr decltype(auto) elementAt(std::integer_sequence<T, t...> c, Index&&, PriorityTag<1>)
   {
     return std::get<Index::value>(std::make_tuple(std::integral_constant<T, t>()...));
   }
 
   template<class Container, class Index>
-  constexpr auto elementAt(Container&& c, Index&& i, PriorityTag<0>)
+  constexpr decltype(auto) elementAt(Container&& c, Index&& i, PriorityTag<0>)
   {
     return c[i];
   }
@@ -131,7 +131,7 @@ namespace Imp {
  * * std::integer_sequence
  */
 template<class Container, class Index>
-constexpr auto elementAt(Container&& c, Index&& i)
+constexpr decltype(auto) elementAt(Container&& c, Index&& i)
 {
   return HybridAlgorithm::Imp::elementAt(std::forward<Container>(c), std::forward<Index>(i), PriorityTag<42>());
 }