interface TheInterface {
mut:
    an_interface() ?
}

struct Implementation {
}

fn (mut i Implementation) an_interface() ? {
    return
}

fn maker() ?TheInterface {
    inner := Implementation{}
    return inner
}

fn do(mut inter TheInterface) string {
    return 'ok'
}

fn test_fn_mut_arg_of_interface() {
    mut inner := maker()?
    ret := do(mut inner)
    println(ret)
    assert ret == 'ok'
}