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Incorrect exhaustiveness warning with GADTs #6395

vicuna opened this Issue May 5, 2014 · 2 comments


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vicuna commented May 5, 2014

Original bug ID: 6395
Reporter: @yallop
Assigned to: @garrigue
Status: closed (set by @xavierleroy on 2017-09-24T15:32:13Z)
Resolution: fixed
Priority: normal
Severity: minor
Target version: 4.03.1+dev
Fixed in version: 4.03.0+dev / +beta1
Category: typing
Child of: #5998
Monitored by: @hcarty

Bug description

Given the following type definitions:

type p = P
type q = Q

type _ t =
X : bool -> p t
| Y : q t

the following code correctly compiles without exhaustiveness warnings

let f : type a. a t * a t -> unit = function
| X _, X _ -> ()
| Y, Y -> ()

but the following code

let g : type a. a t * a t -> unit = function
| x, X false -> ()
| X _, X _ -> ()
| Y, Y -> ()

gives a warning:

Warning 8: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
(Y, X true)


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vicuna commented May 11, 2014

Comment author: @garrigue

Another example which shows that the GADT exhaustiveness check, while sound, is extremely fragile.


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vicuna commented Apr 5, 2016

Comment author: @garrigue

Fixed in 4.03 by the new exhaustuveness/redundancy check for GADTs.
But one could probably other (more involved) cases of asymmetry.

@vicuna vicuna closed this Sep 24, 2017

@vicuna vicuna added the typing label Mar 14, 2019

@vicuna vicuna added this to the 4.03.1 milestone Mar 14, 2019

@vicuna vicuna added the bug label Mar 20, 2019

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