移动语义和复制构造函数

我写了一个程序如下:

#include <iostream>

using namespace std;

class A {
public:
    A() {
    }
    A(A &a) {
        id = a.id;
        cout << "copy constructor" << endl;
    }
    A& operator=(A &other) {
        id = other.id;
        cout << "copy assignment" << endl;
        return *this;
    }
    A(A &&other) {
        id = other.id;
        cout << "move constructor" << endl;
    }
    A& operator=(A &&other) {
        id = other.id;
        cout << "move assignment" << endl;
        return *this;
    }
public:
    int id = 10;
};

A foo() {
    A a;
    return a;
}

int main()
{
    A a;
    A a2(a); // output: copy constructor
    A a3 = a2; // output: copy constructor
    a3 = a2; // output: copy assignment
    A a4 = foo(); // output: 
    a4 = foo(); // output: move assignment
    return 0;
}

我在我的Mac操作系统上编译了它。输出为:

copy constructor
copy constructor
copy assignment
move assignment

我的问题是:

1. 为什么 A a4 = foo(); 是空的吗?我认为它应该调用move构造函数。
2. 为什么 A a3 = a2; 是 copy constructor 而不是 copy assignment ?