Jacques Garrigue wrote:
> The message is maybe not clear enough: for pattern-matching and
> inheritance, you need an "exact" variant type, i.e. a type whose lower
> and upper bounds are identical. 
Indeed - I would suggest that the error include some clearer diagnostics 
that indicate how the variant is 'inexact'.

> The standard workaround is to write:
>
> type c0 = [a | b]
> type 'a c = 'a constraint 'a = [< c0]
>   
This got me further, thanks.  But not far enough... 
Here is a simple example of the problems I am now running into:

type 'a a = [`A of 'a ]
type 'a b = [`B of 'a ]
type    d = [`D ]
type ('a,'b) e = ['b a | 'a b | d]

type ('a,'b) c = ('a,'b) e
    constraint 'a = [< 'b a ]
    constraint 'b = [< 'a b ]

type 'd cc = 'd
    constraint 'd = [< ('a,'b) c]

let is_cc = function
    | #cc -> true
    | _   -> false 
;;

(* complains that cc is not a variant type when trying to define is_cc *)

Jacques Carette