struct A {
    void operator++() const {}
};

void operator++(const A&) {}


int main () {
    const A ca;
    ++ca; // g++ Error (as expected): ambiguous overload for ‘operator++’

    A a;
    ++a; // g++ Warning: "ISO C++ says that these are ambiguous,// even though the worst conversion for the first is better
         // than the worst conversion for the second"
         // candidate 1: void operator++(const A&)
         // candidate 2: void A::operator++() const
}

为什么g只发出警告而不是错误？换句话说,非成员函数如何比成员函数更合适？

谢谢！