error[E0038]: the trait `Foo` cannot be made into an object --> $DIR/issue-38604.rs:14:13 | LL | let _f: Box = | ^^^^^^^^^^^^ `Foo` 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 --> $DIR/issue-38604.rs:2:22 | LL | trait Foo where u32: Q { | --- ^^^^^^^ ...because it uses `Self` as a type parameter | | | this trait cannot be made into an object... error[E0038]: the trait `Foo` cannot be made into an object --> $DIR/issue-38604.rs:15:9 | LL | Box::new(()); | ^^^^^^^^^^^^ `Foo` 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 --> $DIR/issue-38604.rs:2:22 | LL | trait Foo where u32: Q { | --- ^^^^^^^ ...because it uses `Self` as a type parameter | | | this trait cannot be made into an object... = note: required for the cast from `Box<()>` to `Box` error: aborting due to 2 previous errors For more information about this error, try `rustc --explain E0038`.