[Fix] isQuadratic utility function.

dune/matrix-vector/traits/utilities.hh

@@ -92,7 +92,8 @@ namespace MatrixVector {
 
 template <class T>
 constexpr auto isQuadratic() {
-  return Std::bool_constant<Traits::IsQuadratic<std::decay_t<T>>::dimension>();
+  return std::integral_constant<
+      int, Traits::IsQuadratic<std::decay_t<T>>::dimension>();
 }
 
 } // end namespace MatrixVector