partial_swap.h

// Ensures that *__x, *__y and *__z are ordered according to the comparator __c,
// under the assumption that *__y and *__z are already ordered.
template <class _Compare, class _RandomAccessIterator>
inline void __partially_sorted_swap(_RandomAccessIterator __x, _RandomAccessIterator __y,
                                    _RandomAccessIterator __z, _Compare __c) {
  using value_type = typename iterator_traits<_RandomAccessIterator>::value_type;
  bool __r = __c(*__z, *__x);
  value_type __tmp = __r ? *__z : *__x;
  *__z = __r ? *__x : *__z;
  __r = __c(__tmp, *__y);
  *__x = __r ? *__x : *__y;
}

template <class _Compare, class _RandomAccessIterator>
inline void __sort3(_RandomAccessIterator __x1, _RandomAccessIterator __x2,
                    _RandomAccessIterator __x3, _Compare __c) {
  _VSTD::__cond_swap<_Compare>(__x2, __x3, __c);
  _VSTD::__partially_sorted_swap<_Compare>(__x1, __x2, __x3, __c);
}

I think something's missing in __partially_sorted_swap. __tmp is read from once to compare it with __y, but its value is never stored anywhere. So is it missing *__y = __r ? __y : __tmp; at the end?