error[E0038]: the trait `X` cannot be made into an object --> $DIR/object-safety-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 "object safe" 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/object-safety-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 previous error For more information about this error, try `rustc --explain E0038`.