error[E0053]: method `bar` has an incompatible type for trait
  --> $DIR/specialization-broken.rs:15:22
   |
LL | default impl<U> Foo for U
   |              - found this type parameter
...
LL |     fn bar(&self) -> U {
   |                      ^ expected associated type, found type parameter `U`
   |
note: type in trait
  --> $DIR/specialization-broken.rs:8:22
   |
LL |     fn bar(&self) -> impl Sized;
   |                      ^^^^^^^^^^
   = note: expected signature `fn(&_) -> impl Sized`
              found signature `fn(&_) -> U`
help: change the output type to match the trait
   |
LL |     fn bar(&self) -> impl Sized {
   |                      ~~~~~~~~~~

error: method with return-position `impl Trait` in trait cannot be specialized
  --> $DIR/specialization-broken.rs:15:5
   |
LL |     fn bar(&self) -> U {
   |     ^^^^^^^^^^^^^^^^^^
   |
   = note: specialization behaves in inconsistent and surprising ways with return position `impl Trait` in traits, and for now is disallowed

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0053`.