Contravariance and type equations

contravariance_constraint.ml

module type pair = sig type a type b end

module type S = functor (Pair:sig include pair type b = a end) -> sig
  open Pair
  type either = Left of a | Right of b
end

module R = struct
  type a = A
  type b = a
end

module F(Make: S) = struct
  module Mk = Make
  module E = Mk(R)
  let r = E.[Left A; Right A ]
end

module Pair(X:pair) = struct
  open X
  type either = Left of a | Right of b
end

module Ok = F(Pair)