src/test/ui/regions/regions-trait-object-subtyping.rs

trait Dummy { fn dummy(&self); }

fn foo1<'a:'b,'b>(x: &'a mut (dyn Dummy+'a)) -> &'b mut (dyn Dummy+'b) {
    // Here, we are able to coerce
    x
}

fn foo2<'a:'b,'b>(x: &'b mut (dyn Dummy+'a)) -> &'b mut (dyn Dummy+'b) {
    // Here, we are able to coerce
    x
}

fn foo3<'a,'b>(x: &'a mut dyn Dummy) -> &'b mut dyn Dummy {
    // Without knowing 'a:'b, we can't coerce
    x //~ ERROR lifetime bound not satisfied
     //~^ ERROR cannot infer an appropriate lifetime
}

struct Wrapper<T>(T);
fn foo4<'a:'b,'b>(x: Wrapper<&'a mut dyn Dummy>) -> Wrapper<&'b mut dyn Dummy> {
    // We can't coerce because it is packed in `Wrapper`
    x //~ ERROR mismatched types
}

fn main() {}