error[E0277]: the trait bound `impl std::future::Future: std::marker::Copy` is not satisfied
  --> $DIR/issue-55872-2.rs:13:14
   |
LL |     type E = impl Copy;
   |              ^^^^^^^^^ the trait `std::marker::Copy` is not implemented for `impl std::future::Future`
   |
   = note: the return type of a function must have a statically known size

error: type parameter `T` is part of concrete type but not used in parameter list for the `impl Trait` type alias
  --> $DIR/issue-55872-2.rs:15:28
   |
LL |       fn foo<T>() -> Self::E {
   |  ____________________________^
LL | |
LL | |         async {}
LL | |     }
   | |_____^

error: aborting due to 2 previous errors

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