trait X {}

#[derive(Default)]
pub struct XX<'a> {
   _x : std::cell::Cell<Option<Box<X+'a>>>,
   //_y : Option<Box<X+'a>>,
}

pub fn foo<'a>(_l : &'a XX<'a>)  { }

fn main() {
    let g = XX::default();
    foo(&g);
}