error[E0038]: the trait `Foo` cannot be made into an object
  --> $DIR/issue-38604.rs:14:13
   |
LL |     let _f: Box<dyn Foo> =
   |             ^^^^^^^^^^^^ `Foo` 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/issue-38604.rs:2:22
   |
LL | trait Foo where u32: Q<Self> {
   |       ---            ^^^^^^^ ...because it uses `Self` as a type parameter
   |       |
   |       this trait cannot be made into an object...
   = help: only type `()` implements the trait, consider using it directly instead

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 "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/issue-38604.rs:2:22
   |
LL | trait Foo where u32: Q<Self> {
   |       ---            ^^^^^^^ ...because it uses `Self` as a type parameter
   |       |
   |       this trait cannot be made into an object...
   = help: only type `()` implements the trait, consider using it directly instead
   = note: required for the cast from `Box<()>` to `Box<dyn Foo>`

error: aborting due to 2 previous errors

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