Fatal error: exception Ctype.Unify(_)

[vicuna]

Original bug ID: 7695
Reporter: copy
Assigned to: @garrigue
Status: resolved (set by @xavierleroy on 2017-12-28T18:37:46Z)
Resolution: fixed
Priority: normal
Severity: minor
Version: 4.06.0
Target version: 4.07.0+dev/beta2/rc1/rc2
Fixed in version: 4.07.0+dev/beta2/rc1/rc2
Category: typing
Has duplicate: #7766
Child of: #6608
Monitored by: @nojb @gasche

Bug description

The following code produce a compiler error ("Fatal error: exception Ctype.Unify(_)").

let _ =
let arb = QCheck.char in
let gen = QCheck.Gen.int in
{ arb with gen }

Steps to reproduce

1. Install qcheck 0.7 (opam install qcheck)
2. Save the code as test.ml
3. Run: ocamlfind ocamlc -c -package qcheck test.ml

[vicuna]

Comment author: @nojb

Simpler repro
$ rlwrap ocaml
OCaml version 4.06.0

type 'a t = { f : 'a; g : 'a };;

type 'a t = { f : 'a; g : 'a; }

let x = { f = 12; g = 43 };;

val x : int t = {f = 12; g = 43}

{x with f = "hola"};;

Fatal error: exception Ctype.Unify(_)
$

[vicuna]

Comment author: @nojb

Bug introduced in 844016f
(MPR #6608).

[vicuna]

Comment author: @garrigue

Thanks @nojebar.
Here is a PR that fixes the problem: #1541

Note that the output of the test is no there yet, as expect tests do not seem to work currently...

[vicuna]

Comment author: @garrigue

Also, looking at the code, I think that the logic could be better: i.e., first refine the return type for absent fields, and then use this information to type overridden fields.
But this is another story.

[vicuna]

Comment author: @xavierleroy

GPR merged in trunk, will be in 4.07