message: Move from ssize_t to ptrdiff_t
diff: @@ -43,7 +43,7 @@ template<class T, class ITERATOR_T = typename decay_t<T>::iterator>
  class ref_iterator : public std::iterator<
  std::bidirectional_iterator_tag,
  typename decay_t<T>::value_type,
  - ssize_t,
  + ptrdiff_t,
  typename std::remove_pointer<typename decay_t<T>::value_type>::type*,
  typename std::remove_pointer<typename decay_t<T>::value_type>::type&> {
  public:
  @@ -250,7 +250,7 @@ template<class T, class ITERATOR_T = typename decay_t<T>::iterator>
  class filter_iterator : public std::iterator<
  std::forward_iterator_tag,
  typename decay_t<T>::value_type,
  - ssize_t,
  + ptrdiff_t,
  typename std::remove_pointer<typename decay_t<T>::value_type>::type*,
  typename std::remove_pointer<typename decay_t<T>::value_type>::type&> {
  