enum xxx = false;

struct A {

    static if (xxx) {
        ref A opAssign(ref const A a) {
            return this;
        }
    }

    ref A opAssign(int v) {
        return this;
    }

}

struct B {
    A a;
    alias a this;
}

void main() {
    A a;
    B b;
    b = a; // With xxx == true this gives an error, but it compiles fine with xxx == false.
    a = b; // With xxx == false this gives an error, but it compiles fine with xxx == true.
}