error: unconstrained opaque type
  --> $DIR/declared_but_not_defined_in_scope.rs:7:20
   |
LL |     pub type Boo = impl ::std::fmt::Debug;
   |                    ^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `Boo` must be used in combination with a concrete type within the same module

error[E0308]: mismatched types
  --> $DIR/declared_but_not_defined_in_scope.rs:11:5
   |
LL |     pub type Boo = impl ::std::fmt::Debug;
   |                    ---------------------- the expected opaque type
...
LL | fn bomp() -> boo::Boo {
   |              -------- expected `Boo` because of return type
LL |     ""
   |     ^^ expected opaque type, found `&str`
   |
   = note: expected opaque type `Boo`
                found reference `&'static str`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0308`.