module type S = sig
  type t = [`A of int | `B of int]
  val f : [>`A of int] -> int
end

module M : S = struct
  type t = [`A of int | `B of int]
  let f (`A i) = i
end

(*

Error: Signature mismatch:
       Modules do not match:
         sig
           type t = [ `A of int | `B of int ]
           val f : [< `A of 'a ] -> 'a
         end
       is not included in
         S
       Values do not match:
         val f : [< `A of 'a ] -> 'a
       is not included in
         val f : [> `A of int ] -> int

*)