// Invariant for this type is: key_1 == key_2
// no function here breaks it
struct Bar{
    private{
        field other:shared_ptr[Bar];

        field key_1:i32;
        field key_2:i32;
    }

    function new(k:i32):Bar{
        return Bar{key_1 = k, key_2 = k};
    }

    function set_other(self:ref Bar, other:shared_ptr[Bar]){
        self->other = other;
    }

    function foo(bar:ref Bar){
        print(bar->key_1, bar->key_2);
    }

    function copy(target:out Bar, source:ref Bar){
        target->key_1 = source->key_1;

        if ( not source->other.empty() ){
            let other = source->other.get(); //ref Bar
            print(other->key_1, other->key_2); // prints 2 1
        }

        target->key_2 = source->key_2;
    }
};


function foo(){
    var a:shared_ptr[Bar] = shared_ptr::new(Bar::new(1));
    var b:shared_ptr[Bar] = shared_ptr::new(Bar::new(2));

    // copies
    external_call(a, b);

    *a = *b;
}


// compiler does not see the source code for this function
function external_call(a b:shared_ptr[Bar]){
    b->set_other(a);
}