tests/ui/traits/trait-upcasting/deref-upcast-shadowing-lint.rs

//@ check-pass
#![warn(deref_into_dyn_supertrait)]
use std::ops::Deref;

trait Bar<T> {}
trait Foo: Bar<i32> {}

impl<'a> Deref for dyn Foo + 'a {
    //~^ warn: this `Deref` implementation is covered by an implicit supertrait coercion
    type Target = dyn Bar<u32> + 'a;

    fn deref(&self) -> &Self::Target {
        todo!()
    }
}

fn main() {}