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From: Leo White <leo@lpw25.net>
To: caml-list@inria.fr
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Date: Fri, 18 Dec 2015 05:07:44 -0500
In-Reply-To: <5673CF72.4030808@tu-berlin.de>
References: <5673CF72.4030808@tu-berlin.de>
Subject: Re: [Caml-list] polymorphic methods

Hi,

This probably constitutes a bug, seeing as the problem only occurs when using a type alias:

# class foo = object
    method f : 'a 'b. (< x: 'b; ..> as 'a) -> 'b = assert false
  end;;
    class foo : object method f : < x : 'b; .. > -> 'b end

# type ('a, 'b) f_arg = < x : 'b; .. > as 'a;;
type ('a, 'b) f_arg = 'a constraint 'a = < x : 'b; .. >

# class foo = object
    method f : 'a 'b. ('a, 'b) f_arg -> 'b = assert false
  end;;
    Characters 30-76:
      method f : 'a 'b. ('a, 'b) f_arg -> 'b = assert false
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: This expression has type 'a 'b. (< x : 'b; .. > as 'a, 'b) f_arg -> 'b
       but an expression was expected of type
         'c 'b. (< x : 'b; .. > as 'c, 'b) f_arg -> 'b
       The type variable 'd occurs inside 'd

Regards,

Leo