2 Checks for indirect collection of populated `Option`
5 `Option` is like a collection of 0-1 things, so `flatten`
6 automatically does this without suspicious-looking `unwrap` calls.
10 let _ = std::iter::empty::<Option<i32>>().filter(Option::is_some).map(Option::unwrap);
14 let _ = std::iter::empty::<Option<i32>>().flatten();