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(_)
$

Comment author: @nojb

Bug introduced in 844016f
(MPR #6608).

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...

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.

Comment author: @xavierleroy

GPR merged in trunk, will be in 4.07