[Caml-list] deep coercion does not work for some stdlib types

[Ashish Agarwal <agarwal1975@gmail.com>]

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Section 7.9.2 of the manual gives this example:
http://caml.inria.fr/pub/docs/manual-ocaml-400/manual021.html#toc76

# module N : sig
     type t = private int
     val of_int: int -> t
     val to_int: t -> int
   end
  = struct
       type t = int
       let of_int n = assert (n >= 0); n
       let to_int n = n
    end;;

module N :  sig type t = private int val of_int : int -> t val to_int : t
-> int end

Deep coercion to a list of integers works as expected:

# let l = List.map N.of_int [1;2;3];;
val l : N.t list = [1; 2; 3]

# (l :> int list);;
- : int list = [1; 2; 3]

But for arrays it doesn't work:

# let a = Array.of_list l;;
val a : N.t array = [|1; 2; 3|]

# (a :> int array);;
Error: Type N.t array is not a subtype of int array

Is this because the array type does not have a variance annotation? If so,
why doesn't it?

I can get around this by mapping over the elements:

# Array.map (fun (x : N.t) -> (x :> int)) a;;

But am I right that coercions have no run-time cost, as where the mapping
will?

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