warning: opaque type `impl Trait<Assoc = impl Send>` does not satisfy its associated type bounds
  --> $DIR/nested-return-type2.rs:28:24
   |
LL |     type Assoc: Duh;
   |                 --- this associated type bound is unsatisfied for `impl Send`
...
LL | fn foo() -> impl Trait<Assoc = impl Send> {
   |                        ^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(opaque_hidden_inferred_bound)]` on by default
help: add this bound
   |
LL | fn foo() -> impl Trait<Assoc = impl Send + Duh> {
   |                                          +++++

warning: 1 warning emitted