Original bug ID: 7482
Reporter: @mlasson
Assigned to: @alainfrisch
Status: resolved (set by @alainfrisch on 2017-02-20T09:38:42Z)
Resolution: duplicate
Priority: normal
Severity: minor
Target version: 4.06.0 +dev/beta1/beta2/rc1
Category: typing
Duplicate of: #6013
Monitored by: @diml

Bug description

When compiling

  let f = ()
    let () =
      let f = () in (* unused *)
      ()

one gets a "Warning: unused variable f", which is fine.

But if you craft this piece of code in a ppx extension that does
not locate its identifiers (or that locate them at the same place),
the warning will not be triggered.

This is because these checks are implemented using hash tables with
the "located name" used as a key, so the two occurrences of "f" are
seen as used (because the first one is).

( Note that this was "discovered" while investigating this issue LexiFi/landmarks#9 )

Steps to reproduce

You can try by compiling :

let str = [%str
    let f = ()
    let () =
      let f = () in
      ()
   ]

let () =
  let open Ast_mapper in
  register "landmarks"
    (fun _ ->
      { default_mapper with
        structure = (fun _ _ -> str)})

with ocamlfind ocamlopt -package ppx_tools.metaquot -package compiler-libs.common -linkpkg -o ppx_bug ppx_bug.ml (it requires ocamlfind and ppx_tools).

And then use the ppx to compile any file (eg. "test.ml"):
ocamlc -ppx ./ppx_bug -dsource test.ml.
It will output:
let f = ()
let () = let f = () in ()
And not trigger any warning.