error: item constrains opaque type that is not in its signature
  --> $DIR/higher_kinded_params2.rs:26:13
   |
LL |         f = g;
   |             ^
   |
   = note: this item must mention the opaque type in its signature in order to be able to register hidden types
note: this item must mention the opaque type in its signature in order to be able to register hidden types
  --> $DIR/higher_kinded_params2.rs:25:8
   |
LL |     fn successors(&self, mut f: for<'x> fn(&'x ()) -> <&'x A as B>::C) -> Successors<'_> {
   |        ^^^^^^^^^^

error: aborting due to previous error