error[E0038]: the trait `X` cannot be made into an object
  --> $DIR/bounds.rs:7:15
   |
LL | fn f() -> Box<dyn X<U = u32>> {
   |               ^^^^^^^^^^^^^^ `X` cannot be made into an object
   |
note: for a trait to be "dyn-compatible" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
  --> $DIR/bounds.rs:4:13
   |
LL | trait X {
   |       - this trait cannot be made into an object...
LL |     type U: PartialEq<Self>;
   |             ^^^^^^^^^^^^^^^ ...because it uses `Self` as a type parameter

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0038`.