Allow subdomain index set to return index for multidomain entities

dune/grid/multidomaingrid/subdomaingrid/indexsets.hh

@@ -69,7 +69,7 @@ public:
 
   template<typename Entity>
   IndexType index(const Entity& e) const {
-    return index<Entity::codimension>(e);
+    return _mdIndexSet.template index<Entity::codimension>(_grid.domain(),_grid.multiDomainEntity(e));
   }
 
   template<int codim, typename Entity>

dune/grid/multidomaingrid/subdomaingrid/subdomaingrid.hh

@@ -611,8 +611,20 @@ public:
     return EntityWrapper<EntityType::codimension,dimension,const GridImp>(this,mdEntity);
   }
 
-  template<typename EntityType>
-  static const typename MDGrid::template MultiDomainEntity<EntityType>::type& multiDomainEntity(const EntityType& e) {
+  //! specialization for -parent- multidomain entity
+  template <
+      class EntityType,
+      std::enable_if_t<std::is_same_v<typename MDGrid::Traits::template Codim<EntityType::codimension>::Entity,EntityType>, int> = 0>
+  static const EntityType&
+  multiDomainEntity(const EntityType &e) {
+    return e;
+  }
+
+  template <
+      class EntityType,
+      std::enable_if_t<!std::is_same_v<typename MDGrid::Traits::template Codim<EntityType::codimension>::Entity,EntityType>,int> = 0>
+  static const typename MDGrid::Traits::template Codim<EntityType::codimension>::Entity &
+  multiDomainEntity(const EntityType &e) {
     return e.impl().multiDomainEntity();
   }