friendly assignment operator

I'm having difficulty writing a friend operator, checkout the code below. The first operator compiles fine, but the second (the “=” operator) gives me a bunch of errors (first of which says “error C2801: 'operator =' must be a non-static member” in VC++ 2003). Is it illegal to define a friend operator for assignment? Thanks.

Code:

---

class A {
public:

        int x;
};

class B {
public:

        friend A& operator += (A &a, const B &b) {
                a.x = 1;
                return a;
        }

        friend A& operator = (A &a, const B &b) { // is this legal?
                a.x = 2;
                return a;
        }
};

---

Assignment operator can only be a member function of the class.

And in addition, (a method in) B just can't claim to be a friend of A to access its private members. Friendship works the other way: class A has to say it trusts (a method in) B, before B has any right's to A's data.