message: iter_impl template conformance with C++17
diff: @@ -3631,7 +3631,7 @@ This class implements a both iterators (iterator and const_iterator) for the
  iterators in version 3.0.0 (see https://github.com/nlohmann/json/issues/593)
  */
  template<typename BasicJsonType>
  -class iter_impl : public std::iterator<std::bidirectional_iterator_tag, BasicJsonType>
  +class iter_impl
  {
  /// allow basic_json to access private members
  friend iter_impl<typename std::conditional<std::is_const<BasicJsonType>::value, typename std::remove_const<BasicJsonType>::type, const BasicJsonType>::type>;
  @@ -3645,6 +3645,14 @@ class iter_impl : public std::iterator<std::bidirectional_iterator_tag, BasicJso
  "iter_impl only accepts (const) basic_json");
  public:
  +
  + /// The std::iterator class template (used as a base class to provide typedefs) is deprecated in C++17.
  + /// The C++ Standard has never required user-defined iterators to derive from std::iterator.
  + /// A user-defined iterator should provide publicly accessible typedefs named
  + /// iterator_category, value_type, difference_type, pointer, and reference.
  + /// Note that value_type is required to be non-const, even for constant iterators.
  + using iterator_category = std::bidirectional_iterator_tag;
  +
  /// the type of the values when the iterator is dereferenced
  using value_type = typename BasicJsonType::value_type;
  /// a type to represent differences between iterators
  