error: concrete type differs from previous defining opaque type use
  --> $DIR/not_a_defining_use.rs:24:1
   |
LL | fn four<T: Debug, U: Bar>(t: T) -> Two<T, U> {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `(T, i8)`, got `(T, <U as Bar>::Blub)`
   |
note: previous use here
  --> $DIR/not_a_defining_use.rs:10:1
   |
LL | fn three<T: Debug, U>(t: T) -> Two<T, U> {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0277]: `T` doesn't implement `Debug`
  --> $DIR/not_a_defining_use.rs:7:18
   |
LL | type Two<T, U> = impl Debug;
   |                  ^^^^^^^^^^ `T` cannot be formatted using `{:?}` because it doesn't implement `Debug`
   |
   = note: required because of the requirements on the impl of `Debug` for `(T, i8)`
help: consider restricting type parameter `T`
   |
LL | type Two<T: std::fmt::Debug, U> = impl Debug;
   |           +++++++++++++++++

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0277`.