::map_rank

#include <map>
#include <type_traits>

namespace details {

  template<typename T>
  struct is_map : std::false_type { };

  template<typename Key, typename Value, typename Compare, typename Allocator>
  struct is_map<std::map<Key, Value, Compare, Allocator>> : std::true_type { };

  template<std::size_t Value>
  using constant = std::integral_constant<std::size_t, Value>;

  template<typename T, bool IsMap = is_map<T>::value>
  struct map_rank {
    using mapped_type = typename T::mapped_type;
    using mapped_type_rank = typename map_rank<mapped_type>::type;
    using type = constant<mapped_type_rank::value + 1>;
  };

  template<typename T>
  struct map_rank<T, false> {
    using type = constant<0>;
  };

}

template<typename Map>
struct map_rank : details::map_rank<Map>::type { };