warning: this `Deref` implementation is covered by an implicit supertrait coercion
  --> $DIR/deref-lint.rs:9:1
   |
LL | impl<'a> Deref for dyn 'a + B {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `dyn B` implements `Deref<Target = dyn A>` which conflicts with supertrait `A`
...
LL |     type Target = dyn A;
   |     -------------------- target type is a supertrait of `dyn B`
   |
   = help: consider removing this implementation or replacing it with a method instead
   = note: `#[warn(deref_into_dyn_supertrait)]` on by default

warning: 1 warning emitted