error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
  --> $DIR/issue-87340.rs:8:6
   |
LL | impl<T> X for () {
   |      ^ unconstrained type parameter

error[E0282]: type annotations needed
  --> $DIR/issue-87340.rs:11:23
   |
LL |     fn f() -> Self::I {}
   |                       ^^ cannot infer type for type parameter `T`

error[E0282]: type annotations needed
  --> $DIR/issue-87340.rs:11:15
   |
LL |     fn f() -> Self::I {}
   |               ^^^^^^^ cannot infer type for type parameter `T`

error: aborting due to 3 previous errors

Some errors have detailed explanations: E0207, E0282.
For more information about an error, try `rustc --explain E0207`.