warning: opaque type `impl Trait<Assoc = Sendable>` does not satisfy its associated type bounds
  --> $DIR/nested-return-type3-tait.rs:19:24
   |
LL |     type Assoc: Duh;
   |                 --- this associated type bound is unsatisfied for `Sendable`
...
LL | fn foo() -> impl Trait<Assoc = Sendable> {
   |                        ^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(opaque_hidden_inferred_bound)]` on by default

warning: 1 warning emitted