为什么返回不复制对象？

以下代码只打印A::A()，而不打印A::A(const A&)或operator=。为什么？

struct A
{
A()               { cout << "A::A()" << endl; }
A(const A& value) { cout << "A::A(const A&)" << endl; }
A& operator=(const A& newValut)
{
cout << "A::operator=" << endl; 
return *this;
}
};
A foo()
{
A a;      //Ok, there we have to create local object by calling A::A().
return a; //And there we need to copy it, otherwise it will be destroyed
//because it's local object. But we don't.
}
int main()
{
A aa = foo(); //Also there we need to put result to the aa
//by calling A::A(const A&), but we don't.
}

所以这个代码必须打印

A::A()
A::A(const A&)
A::A(const A&)

但事实并非如此。为什么？

我建议在没有优化的情况下，g++下没有foo()的内联