The variable "__y" in _Rb_tree_increment (src/tree.cc) is always equal to "__x->_M_parent. Therefore the condition means (which is alwys true):

if (__x->_M_right != __x->_M_parent)

The source:

  _Rb_tree_node_base*
  _Rb_tree_increment(_Rb_tree_node_base* __x) throw ()
  {
    if (__x->_M_right != 0) 
      {
        __x = __x->_M_right;
        while (__x->_M_left != 0)
          __x = __x->_M_left;
      }
    else 
      {
        _Rb_tree_node_base* __y = __x->_M_parent;      // y == x->parent
        while (__x == __y->_M_right) 
          {
            __x = __y;
            __y = __y->_M_parent;                 // y == x->parent
          }
        if (__x->_M_right != __y)    // if( x->right != x->parent )
          __x = __y;
      }
    return __x;
  }