[bugfix] make AddRefTypeEvaluator work in tranformTuple

As documented, tranformTuple should support AddRefTypeEvaluator. As
std::make_tuple will remove references (using std::decay) from the
funtor's return type, the returned tuple would contain copies of the
values. This is not the intended behavior.

This patch adds AddRefTypeEvaluator to the utilitytest and fixes the
return type of transformTuple.

dune/common/test/utilitytest.cc

@@ -27,7 +27,6 @@ struct Counter {
   int result_;
 };
 
-
 int main(int, char**)
 {
 
@@ -59,6 +58,7 @@ int main(int, char**)
   long l = 4;
   char c = 's';
 
+  typedef std::tuple<int,char,long,char> Tuple1;
   typedef std::tuple<int&,char&,long&,char&> RefTuple1;
   typedef std::tuple<int*,char*,long*,char*> PointerTuple1;
   static_assert((std::is_same<PointerTuple1,
@@ -67,7 +67,9 @@ int main(int, char**)
                      "RefTuple1 with added pointers should be the same as "
                      "PointerTuple1, but it isn't!");
 
+  Tuple1 t1(i,c,l,c);
   RefTuple1 refs(i, c, l, c);
+  DUNE_UNUSED RefTuple1 refs2(Dune::transformTuple<Dune::AddRefTypeEvaluator>(t1));
   PointerTuple1 pointers1
     (Dune::transformTuple<Dune::AddPtrTypeEvaluator>(refs));
   if(&i != std::get<0>(pointers1) || &c != std::get<1>(pointers1) ||

dune/common/tupleutility.hh

@@ -79,10 +79,10 @@ namespace Dune {
 
 #ifndef DOXYGEN
   template<class Tuple, class Functor, std::size_t... I>
-  inline auto genericTransformTupleBackendImpl(Tuple& t, Functor& f, const Std::index_sequence<I...>& ) ->
-    decltype(std::make_tuple(f(std::get<I>(t))...)) const
+  inline auto genericTransformTupleBackendImpl(Tuple& t, Functor& f, const Std::index_sequence<I...>& )
+    -> std::tuple<decltype(f(std::get<I>(t)))...>
   {
-    return std::make_tuple(f(std::get<I>(t))...);
+    return std::tuple<decltype(f(std::get<I>(t)))...>(f(std::get<I>(t))...);
   }
 
   template<class... Args, class Functor>