module type Functor = sig
    type 'a t
    val fmap : ('a -> 'b) -> 'a t -> 'b t
  end

let makeFunctor (type s) ~fmap =
  let module S = struct
    type 'a t = s
    let fmap = fmap
  end
  in (module S : Functor with type t = s)

module MyF = (val makeFunctor ~fmap:(fun f v -> match (f, v) with
                                      | (f, (Some x)) -> Some (f x)
                                      | (_, None) -> None))

=>
Error: In this `with' constraint, the new definition of t
       does not match its original definition in the constrained signature:
       Type declarations do not match: type t is not included in type 'a t
       They have different arities.