error: opaque type's hidden type cannot be another opaque type from the same scope
  --> $DIR/issue-70877.rs:31:12
   |
LL |     return func(&"oof");
   |            ^^^^^^^^^^^^ one of the two opaque types used here has to be outside its defining scope
   |
note: opaque type whose hidden type is being assigned
  --> $DIR/issue-70877.rs:28:19
   |
LL | fn oof(_: Foo) -> impl std::fmt::Debug {
   |                   ^^^^^^^^^^^^^^^^^^^^
note: opaque type being used as hidden type
  --> $DIR/issue-70877.rs:4:15
   |
LL | type FooRet = impl std::fmt::Debug;
   |               ^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error