`Projection` assumption have stronger requirements than goals

[lcnr]

cc rust-lang/trait-system-refactor-initiative#1

fn next<T: Iterator<Item = U>, U>(t: &mut T) -> Option<U> {
    t.next()
}

fn foo<T: Iterator>(t: &mut T) {
    let _: Option<T::Item> = next(t);
}