[Caml-list] Clarification needed: use of "as" in patterns (with GADTs)

[Sebastien Furic <programming.languages@furic.org>]

  Hello,

  Would someone be kind enough to explain me what's going on with the 
following code:

type empty
and nonempty
type ('a, _) my_list =
   | Nil: ('a, empty) my_list
   | Cons: 'a * ('a, 'b) my_list -> ('a, nonempty) my_list

(* Works fine *)
let rec max = function
   | Cons (x, Nil) -> x
   | Cons (x, Cons (x', xs)) when x <= x' -> max (Cons (x', xs))
   | Cons (x, Cons (_, xs)) -> max (Cons (x, xs))

(* Fails *)
let rec max = function
   | Cons (x, Nil) -> x
   | Cons (x, (Cons (x', _) as xs)) when x <= x' -> max xs
   | Cons (x, Cons (_, xs)) -> max (Cons (x, xs));;

Characters 97-99:
   | Cons (x, (Cons (x', _) as xs)) when x <= x' -> max xs
                                                        ^^
Error: This expression has type ('a, nonempty) my_list
        but an expression was expected of type ('a, nonempty) my_list
        This instance of nonempty is ambiguous:
        it would escape the scope of its equation

  I remember having seen similar issues in the past, involving "as" and 
polymorphic variants (but I can't find it in the archives). Is it the 
same issue? Why does Ocaml need to "break the continuity" of types in 
presence of "as"?
  BTW, what is the recommended way to write the code above (I want to 
avoid having to reconstruct the list)?

  Cheers,

  Sébastien.