Strange signatures for IndexSet and IdSet methods taking an entity

IndexSet and IdSet have implement (nearly) all methods taking an entity twice, e.g.,

Index index ( const Entity &entity ) const
{
  return index< Entity::codimension >( entity );
}

template< int cc >
Index index ( const typename GridImp::template Codim< cc >::Entity &entity ) const
{
  return asImp().index< cc >( entity );
}

The implementation then only implements the method index< cc >. This has the downside, that the implementation has to use IndexSet::index to provide the first version, if not casted down to the interface.

As far as I know, the purpose of this technique is to ensure the type Entity coinsides with typename GridImp::template Codim< cc >::Entity. Today, this could also be done by an SFINAE:

template< class Entity , std::enable_if_t< std::is_same< Entity, typename GridImp::template Codim< Entity::codimension >::Entity >::value, int > = 0 >
Index index ( const Entity &entity ) const
{
  asImp().index( entity );
}

This way, the interface would still protect the method index from being called with a wrong entity type.

I propose to use the SFINAE version, here. However, this will break implementations, unless we use another SFINAE trick to detect the implementation (and use proper deprecation).