#[derive(Default)]
struct Foo;

fn bar<S: IntoIterator<Item = Foo>>(s: Option<S>) {
    let f = s.unwrap_or(vec![Foo::default()].into_iter());
    baz(f)
}

fn baz<S: IntoIterator<Item = Foo>>(s: S) {}

fn main() {
    println!("Hello, world!");
}