Unhandled exception from List.nth when using -short-paths with ill-typed code

[vicuna]

Original bug ID: 7543
Reporter: @jberdine
Assigned to: @Octachron
Status: resolved (set by @mshinwell on 2017-06-09T17:03:19Z)
Resolution: duplicate
Priority: normal
Severity: crash
Version: 4.04.1
Category: typing

Bug description

It seems that ill-typed code can cause -short-paths to raise instead of detecting the error.

Steps to reproduce

$ cat repro.ml
open Base

let e = Set.empty (module Comparator.Poly)
$ ocamlc -I ~/.opam/4.04.1/lib/base repro.ml
File "repro.ml", line 3, characters 26-41:
Error: Signature mismatch:
       ...
       Type declarations do not match:
         type 'a t = 'a
       is not included in
         type t = Base.Comparator.Poly.t
       They have different arities.
$ ocamlc -I ~/.opam/4.04.1/lib/base repro.ml -short-paths
Fatal error: exception Failure("nth")
$ OCAMLRUNPARAM=b ocamlc -I ~/.opam/4.04.1/lib/base repro.ml -short-paths
Fatal error: exception Failure("nth")
Raised at file "pervasives.ml", line 32, characters 17-33
Called from file "typing/printtyp.ml", line 235, characters 15-30
Called from file "typing/printtyp.ml", line 489, characters 43-62
Called from file "typing/printtyp.ml", line 803, characters 8-21
Called from file "typing/printtyp.ml", line 905, characters 13-38
Called from file "typing/printtyp.ml", line 908, characters 27-72
Called from file "format.ml", line 1187, characters 4-20
Called from file "format.ml", line 1200, characters 32-48
Called from file "format.ml", line 1200, characters 32-48
Called from file "format.ml", line 1187, characters 4-20
Called from file "format.ml", line 1248, characters 20-38
Called from file "typing/includemod.ml", line 529, characters 6-343
Called from file "format.ml", line 1187, characters 4-20
Called from file "format.ml", line 1248, characters 20-38
Called from file "utils/misc.ml", line 28, characters 20-27
Re-raised at file "utils/misc.ml", line 28, characters 50-57
Called from file "utils/misc.ml", line 40, characters 10-14
Re-raised at file "utils/misc.ml", line 42, characters 38-45
Called from file "format.ml", line 1187, characters 4-20
Called from file "format.ml", line 1248, characters 20-38
Called from file "format.ml", line 1187, characters 4-20
Called from file "format.ml", line 1248, characters 20-38
Called from file "utils/misc.ml", line 28, characters 20-27
Re-raised at file "utils/misc.ml", line 28, characters 50-57
Called from file "utils/misc.ml", line 40, characters 10-14
Re-raised at file "utils/misc.ml", line 42, characters 38-45
Called from file "format.ml", line 1187, characters 4-20
Called from file "format.ml", line 1248, characters 20-38
Called from file "parsing/location.ml" (inlined), line 415, characters 2-28
Called from file "typing/typemod.ml", line 1830, characters 13-67
Called from file "parsing/location.ml", line 384, characters 14-19
Called from file "parsing/location.ml", line 447, characters 12-28
Re-raised at file "parsing/location.ml", line 450, characters 12-23
Re-raised at file "parsing/location.ml", line 450, characters 12-23
Re-raised at file "parsing/location.ml", line 450, characters 12-23
Re-raised at file "parsing/location.ml", line 450, characters 12-23
Re-raised at file "parsing/location.ml", line 450, characters 12-23
Called from file "parsing/location.ml" (inlined), line 454, characters 31-61
Called from file "driver/main.ml", line 190, characters 4-35
Called from file "utils/timings.ml", line 69, characters 10-13
Called from file "driver/main.ml", line 194, characters 2-28
$

[vicuna]

Comment author: @jberdine

Sorry, the repro was unnecessarily not self-contained. Here is a smaller one:

module type S = sig type t end

let f ((module M) : (module S with type t = unit)) = ()

module N = struct type 'a t = 'a end

let x = f (module N)

Also, I checked that it repros with the 4.05.0+trunk opam switch.

[vicuna]

Comment author: @Octachron

Thanks for the report (and the reduced test case)! I have proposed a minimal fix to the issue in #1186 .

[vicuna]

Comment author: @mshinwell

Please continue discussion on the Github issue.