Overloaded operators obey the rules for syntax specified in clause 5, but the requirements of operand type, lvalue, and evaluation order are replaced by the rules for function call.

• 可以看到求值次数并不在否定之列，所以在这一点上标准对于内置+=运算符的说明（我在前面的贴子中引用过的一句话）仍然适用于重载运算符的情况。