[bugfix] Use std::decay_t to make IsIntegralConstant more robust

dune/solvers/common/algorithm.hh

@@ -4,6 +4,7 @@
 #define DUNE_SOLVERS_COMMON_ALGORITHM_HH
 
 
+#include <dune/common/typeutilities.hh>
 
 namespace Dune {
 namespace Solvers {
@@ -33,17 +34,22 @@ struct StaticForLoop<ST, end, end>
 
 // Check if T is an integral constant
 template<class T>
-struct IsIntegralConstant
+struct IsIntegralConstantHelper
 {
     static const bool value = false;
 };
 
 template<class T, T t>
-struct IsIntegralConstant<std::integral_constant<T, t>>
+struct IsIntegralConstantHelper<std::integral_constant<T, t>>
 {
     static const bool value = true;
 };
 
+// Check if T is an integral constant
+template<class T>
+struct IsIntegralConstant : public IsIntegralConstantHelper<std::decay_t<T>>
+{};
+
 } //end namespace Imp
 
 
@@ -139,6 +145,14 @@ constexpr auto staticSize(const T*, const PriorityTag<2>&)
   return {};
 }
 
+// Try if tuple_size is implemented for class
+template<class T, int i>
+constexpr auto staticSize(const Dune::FieldVector<T, i>*, const PriorityTag<3>&)
+  -> decltype(std::integral_constant<std::size_t,i>())
+{
+  return {};
+}
+
 template<class T>
 constexpr std::false_type hasStaticSize(const T* t, const PriorityTag<0>& p)
 {
@@ -200,7 +214,7 @@ struct StaticSize :
  */
 template<class T,
     std::enable_if_t<HasStaticSize<T>::value, int> = 0>
-auto hybridSize(const T& t)
+constexpr auto hybridSize(const T& t)
 {
   return Imp::staticSize((T*)(nullptr), PriorityTag<42>());
 }
@@ -221,7 +235,7 @@ auto hybridSize(const T& t)
  */
 template<class T,
     std::enable_if_t<not HasStaticSize<T>::value, int> = 0>
-auto hybridSize(const T& t)
+constexpr auto hybridSize(const T& t)
 {
   return t.size();
 }