module type S = sig type t end
module N : sig type 'a t = 'a end
val f : (module S with type t = unit) -> unit = <fun>
Line 1, characters 19-20:
1 | let () = f (module N);;
                       ^
Error: Signature mismatch:
       Modules do not match: sig type 'a t = 'a end is not included in S
       Type declarations do not match:
         type 'a t = 'a
       is not included in
         type t
       They have different arities.