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