Assertion error with implicit of abstract module type

[yallop]

# module F(X : sig module type T end) =
struct
  let f (implicit Y:X.T) () = ()
  let _ = f ()
end
;;
Fatal error: exception File "typing/implicitsearch.ml", line 400, characters 26-32: Assertion failed

[let-def]

I don't know how to interpret search for an abstract module type.
Of course, a clean error message should be provided. Or maybe there is a cleaner way of handling this case?

Maybe treating X.T path as a witness for a module type and searching for implicits in scope having this name as type?

This would allow:

module F(X : sig module type T module M : T end) =
struct
  let f (implicit Y:X.T) () = ()
  implicit module M = X.M
  let _ = f ()
end