Operator Overloading - General Questions

Operator overloading can change an operator's:

Which operator must be overloaded as a member function?

What will be the output of the program?

#include <iostream>
using namespace std;
struct M {
    int v;
    bool operator==(const M &o) const { return v == o.v; }
};
int main() {
    M a{3}, b{3}, c{4};
    cout << (a == b) << (a == c) << endl;
    return 0;
}

Which operators cannot be overloaded in C++?

What will be the output of the program?

#include <iostream>
using namespace std;
struct A {
    int d[3] = {7, 8, 9};
    int &operator[](int i) { return d[i]; }
};
int main() {
    A a;
    a[1] = 50;
    cout << a[0] + a[1] << endl;
    return 0;
}