Original bug ID: 6987
Reporter: mandrykin
Assigned to: @garrigue
Status: resolved (set by @garrigue on 2017-03-14T07:18:14Z)
Resolution: fixed
Priority: low
Severity: minor
Platform: x86_64
OS: Linux (3.2.0)
OS Version: Ubuntu 12.04
Version: 4.02.3
Target version: undecided
Fixed in version: 4.05.0 +dev/beta1/beta2/beta3/rc1
Category: typing
Monitored by: @gasche

Bug description

The following code:

type 'a t = V1 of 'a

type ('c,'t) pvariant = [ `V of ('c * 't t) ]

class ['c] clss =
object
method mthod : 't . 'c -> 't t -> ('c, 't) pvariant = fun c x ->
`V (c, x)
end

let f2 = fun o c x -> match x with | V1 _ -> x

let rec f1 o c x =
match (o :> _ clss)#mthod c x with
| `V c -> f2 o c x

Is rejected with a strange inclusion error (of a value within itself):

Error: Values do not match:
val f1 :
< mthod : 't. 'a -> 't t -> [< ('a, 't) pvariant ]; .. > ->
'a -> 'b t -> 'b t
is not included in
val f1 :
< mthod : 't. 'a -> 't t -> [< ('a, 't) pvariant ]; .. > ->
'a -> 'b t -> 'b t

However, after substituting the type ('c, 't) pvariant' directly into the type of mthod' the function `f1' is assigned essentially the same type:

f1 : < mthod : 't. 'a -> 't t -> [< `V of 'a * 't t ]; .. > -> 'a -> 'b t -> 'b t

An attempt to call the function `f1' in the following way:

let v' = f1 (object method mthod = fun c (V1 x) -> `V (c, V1 x) end) 0 (V1 0)

casues an error message about a universal variable escaping its scope:

Error: This expression has type
< mthod : 'a -> 'b t -> [> V of int * 'c t ] > but an expression was expected of type < mthod : 't. 'a -> 't t -> [< V of int * 't0 t ]; .. >
The universal variable 't would escape its scope

The function can be successfully called with an object inherited form int clss'. I'm not sure if the initial function f1' should be accepted or what the appropriate error message for it should be, though (in practice the coercion :> _ clss' can be removed or replaced with :> _ #clss', which solves the problem).

File attachments

• suspicious_error.ml