Gcc list node Is complex swap implementation complicated?

I am using SGI STL (GCC) as a reference for a custom library, and while digging into I std::list::swap()

came across the following implementation,

Note. This method does not properly handle neighbors.

// namespace std {
// namespace __detail {
void
_List_node_base::swap(_List_node_base& __x, _List_node_base& __y) throw()
{
  if ( __x._M_next != &__x )
    {
      if ( __y._M_next != &__y )
        {
          // Both __x and __y are not empty.
          std::swap(__x._M_next,__y._M_next);
          std::swap(__x._M_prev,__y._M_prev);
          __x._M_next->_M_prev = __x._M_prev->_M_next = &__x;
          __y._M_next->_M_prev = __y._M_prev->_M_next = &__y;
        }
      else
        {
          // __x is not empty, __y is empty.
          __y._M_next = __x._M_next;
          __y._M_prev = __x._M_prev;
          __y._M_next->_M_prev = __y._M_prev->_M_next = &__y;
          __x._M_next = __x._M_prev = &__x;
        }
    }
  else if ( __y._M_next != &__y )
    {
      // __x is empty, __y is not empty.
      __x._M_next = __y._M_next;
      __x._M_prev = __y._M_prev;
      __x._M_next->_M_prev = __x._M_prev->_M_next = &__x;
      __y._M_next = __y._M_prev = &__y;
    }
}

      

        
        
        
      

    

It looks as if it were simplified,

void
_List_node_base::swap(_List_node_base& __x, _List_node_base& __y) throw()
{
  _List_node_base* __xnext = __x._M_next;
  _List_node_base* __xprev = __x._M_prev;
  _List_node_base* __ynext = __y._M_next;
  _List_node_base* __yprev = __y._M_prev;

  __xnext->_M_prev = __xprev->_M_next = &__y;
  __ynext->_M_prev = __yprev->_M_next = &__x;
  std::swap(__x._M_next,__y._M_next);
  std::swap(__x._M_prev,__y._M_prev);
}

      

        
        
        
      

    

I've tested this for all cases (empty / empty, empty / non-empty, etc.) including __x

and __y

referencing the same node and it seems to work trusting the SGI codebase makes me doubt it.

So my question is: is this correct? And if so, can the longer version be used?

Thank.