fn want_slice(v: [int]/&) -> int {
    let mut sum = 0;
    for vec::each(v) { |i| sum += i; }
    ret sum;
}

fn has_mut_vec(+v: @[mut int]/~) -> int {
    want_slice(*v) //! ERROR illegal borrow unless pure: creating immutable alias to aliasable, mutable memory
        //!^ NOTE impure due to access to impure function
}

fn main() {
    assert has_mut_vec(@[mut 1, 2, 3]/~) == 6;
}