交换没有临时变量，警告错误，为什么？

Question

警告：'left'上的操作可能未定义[-Wsequence-point] right = right + left - （left = right）;

{
    int right = 5, left = 14;
    std::cout << right << " " << left << std::endl;
    right = right + left - (left = right);
    std::cout << right << " " << left << std::endl;
}