Shadowing in a branch of a GADT match breaks unused variable warning

[vicuna]

Original bug ID: 7115
Reporter: @sliquister
Assigned to: @alainfrisch
Status: closed (set by @xavierleroy on 2017-09-24T15:31:43Z)
Resolution: fixed
Priority: normal
Severity: minor
Version: 4.02.3
Fixed in version: 4.03.0+dev / +beta1
Category: typing
Monitored by: @gasche

Bug description

The following code:

type t = A : t
let _apply_implementation t ~x =
match t with
| A -> let x = () in x

should complain about "unused variable x" but doesn't.
If instead [t] is defined as [type t = A], or if the two [x] variables are given different names, then we get:

$ ocamlopt -w +a /tmp/a.ml
File "/tmp/a.ml", line 2, characters 29-30:
Warning 27: unused variable x.

as expected.

[vicuna]

Comment author: @alainfrisch

Probably related to the function duplicate_ident_types in typecore.ml, which says:

    (* XXX This will mark the value as being used;
       I don't think this is what we want *)

[vicuna]

Comment author: @alainfrisch

Fix committed to trunk (4e8cb78). 'unused open' was similarly affected. I still don't understand the need to lookup the environment for existing bindings whose name match bound variables...

[vicuna]

Comment author: @sliquister

Thanks!