error[E0053]: method `bar` has an incompatible type for trait
  --> $DIR/specialization-broken.rs:16:22
   |
LL | default impl<U> Foo for U
   |              - this type parameter
...
LL |     fn bar(&self) -> U {
   |                      ^
   |                      |
   |                      expected associated type, found type parameter `U`
   |                      help: change the output type to match the trait: `impl Sized`
   |
note: type in trait
  --> $DIR/specialization-broken.rs:9:22
   |
LL |     fn bar(&self) -> impl Sized;
   |                      ^^^^^^^^^^
   = note: expected signature `fn(&U) -> impl Sized`
              found signature `fn(&U) -> U`

error: aborting due to previous error

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