[integer sequence] mark make_{index,integer}_sequence and index_sequence_for as DUNE_CONSTEXPR

dune/common/std/utility.hh

@@ -115,7 +115,7 @@ namespace Dune
      *  \tparam  N  requested size of index sequence
      */
     template< std::size_t N >
-    static inline typename make_index_sequence_impl< N >::type make_index_sequence ()
+    static DUNE_CONSTEXPR inline typename make_index_sequence_impl< N >::type make_index_sequence ()
     {
       return typename make_index_sequence_impl< N >::type();
     }
@@ -134,7 +134,7 @@ namespace Dune
      *  \tparam  N  requested size of integer sequence
      */
     template< class T, T N >
-    static inline typename make_index_sequence_impl< N >::type::template rebind< T >::type
+    static DUNE_CONSTEXPR inline typename make_index_sequence_impl< N >::type::template rebind< T >::type
     make_integer_sequence ()
     {
       return typename make_index_sequence_impl< N >::type::template rebind< T >::type();
@@ -153,7 +153,7 @@ namespace Dune
      *  \tparam  ...T  a type parameter pack
      */
     template< class... T >
-    static inline typename make_index_sequence_impl< sizeof...( T ) >::type
+    static DUNE_CONSTEXPR inline typename make_index_sequence_impl< sizeof...( T ) >::type
     index_sequence_for ()
     {
       return typename make_index_sequence_impl< sizeof...( T ) >::type();