error[E0277]: the trait bound `(): Foo` is not satisfied
  --> $DIR/trait-failure.rs:9:17
   |
LL |     let x: impl Foo = W(());
   |                 ^^^ the trait `Foo` is not implemented for `()`
   |
   = help: the trait `Foo` is implemented for `W<T>`
note: required for `W<()>` to implement `Foo`
  --> $DIR/trait-failure.rs:6:9
   |
LL | impl<T> Foo for W<T> where T: Foo {}
   |         ^^^     ^^^^          --- unsatisfied trait bound introduced here

error[E0277]: the trait bound `(): Foo` is not satisfied
  --> $DIR/trait-failure.rs:11:19
   |
LL |     let x: W<impl Foo> = W(());
   |                   ^^^ the trait `Foo` is not implemented for `()`
   |
   = help: the trait `Foo` is implemented for `W<T>`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0277`.