error[E0277]: the trait bound `Self: Eq` is not satisfied
  --> $DIR/wf-trait-default-fn-arg.rs:11:23
   |
LL | struct Bar<T:Eq+?Sized> { value: Box<T> }
   |              -- required by this bound in `Bar`
...
LL |     fn bar(&self, x: &Bar<Self>) {
   |                       ^^^^^^^^^ the trait `Eq` is not implemented for `Self`
   |
help: consider further restricting `Self`
   |
LL |     fn bar(&self, x: &Bar<Self>) where Self: Eq {
   |                                  ^^^^^^^^^^^^^^

error: aborting due to previous error

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