[Caml-list] This ground coercion is not principal?

[bob zhang <bobzhang1988@gmail.com>]

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Hi List,
  I have came across a corner case in ocaml typing here,
The source code is pasted below(extracted from a large code base), it
triggers the warning 18.
My question, is there any elegant solution without introducing local types?

------------------------------------------------------------------------------------------
type ant = [`Ant of string]
type 'a mlist =
    [ `LNil
    | `LCons of ('a * 'a mlist )
    | ant]
let a = object(self)
  method ant : ant -> ant=
    fun (`Ant a)  ->
      `Ant a
  method mlist: 'all_a0 'all_b0 .
      ('self_type -> 'all_a0 -> 'all_b0) ->
        'all_a0 mlist -> 'all_b0 mlist=
          fun (type t) mf_a  ->
            function
              | `LNil ->  `LNil
              | `LCons (a0,a1) ->
                  let a0 = mf_a self a0 in
                  let a1 = self#mlist mf_a a1 in `LCons (a0, a1)
              | #ant as a0 -> (((* Obj.magic *) self#ant a0) :> (* 'all_b0
*) t mlist)
end






















-- 
Regards
-- Bob

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