error[E0277]: the trait bound `std::result::Result<(), _>: Future` is not satisfied
  --> $DIR/lifetime-elision-return-type-trait.rs:8:13
   |
LL | fn foo() -> impl Future<Item=(), Error=Box<dyn Error>> {
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Future` is not implemented for `std::result::Result<(), _>`
   |
   = note: the return type of a function must have a statically known size

error: aborting due to previous error

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