cpp-useful-iterators/include/iterator/detail/recursive_iterator_traits.hpp

#pragma once

namespace iterator { namespace detail {
  struct terminal_layer_tag_t;
  struct continue_layer_tag_t;

  template <typename> struct void_t { using type = void; };

  template <typename IterType>
  using value_iterator = decltype(std::begin(*std::declval<IterType>()));

  template <typename IterType>
  using mapped_iterator =
      decltype(std::begin(std::declval<IterType>()->second));

  /**
   * @class next_layer_type
   * @breif A template metaprogramming type for unifying associative and
   * non-associative containers.
   */
  template <typename V, typename Tag> struct next_layer_type {
    using type = std::tuple<V>;
  };
  template <typename V> struct next_layer_type<V, continue_layer_tag_t> {
    using type = V;
  };
}}