std::map::推論補助(C++17)

namespace std {
  // 説明用の型
  template <class InputIterator>
  using iter_key_t = remove_const_t<typename iterator_traits<InputIterator>::value_type::first_type>;
  template <class InputIterator>
  using iter_val_t = typename iterator_traits<InputIterator>::value_type::second_type;
  template <class InputIterator>
  using iter_to_alloc_t = pair<add_const_t<typename iterator_traits<InputIterator>::value_type::first_type>,
                               typename iterator_traits<InputIterator>::value_type::second_type>;

  template <class InputIterator,
            class Compare = less<iter_key_t<InputIterator>>,
            class Allocator = allocator<iter_to_alloc_t<InputIterator>>>
  map(InputIterator, InputIterator, Compare = Compare(), Allocator = Allocator())
    -> map<iter_key_t<InputIterator>, iter_val_t<InputIterator>, Compare, Allocator>; // (1)

  template <class Key,
            class T,
            class Compare = less<Key>,
            class Allocator = allocator<pair<const Key, T>>>
  map(initializer_list<pair<Key, T>>, Compare = Compare(), Allocator = Allocator())
    -> map<Key, T, Compare, Allocator>;                                               // (2)

  template <class InputIterator, class Allocator>
  map(InputIterator, InputIterator, Allocator)
    -> map<iter_key_t<InputIterator>, iter_val_t<InputIterator>,
           less<iter_key_t<InputIterator>>, Allocator>;                               // (3)

  template <class Key, class T, class Allocator>
  map(initializer_list<pair<Key, T>>, Allocator)
    -> map<Key, T, less<Key>, Allocator>;                                             // (4)
}