OCaml 4.14 infinit loops while building odoc-parser

[kit-ty-kate]

Reproductible with a simple opam install odoc-parser. e.g. with docker:

$ docker run --rm -it ocaml/opam:debian-11-ocaml-4.14
opam@4bca9a9111d3:~$ opam install odoc-parser
The following actions will be performed:
  - install ocamlfind   1.9.1  [required by astring]
  - install dune        2.9.1  [required by odoc-parser]
  - install ocamlbuild  0.14.0 [required by astring]
  - install result      1.5    [required by odoc-parser]
  - install topkg       1.0.4  [required by astring]
  - install astring     0.8.5  [required by odoc-parser]
  - install odoc-parser 0.9.0
[...]

The file it infinit loops on is src/token.ml: https://github.com/ocaml-doc/odoc-parser/blob/626ccc9e319d7d3cc67251425ddbb48e33bc2de8/src/token.ml

[lthls]

Minimal repro:

type style = [ `Bold | `Italic ]
type t = [ `Begin_style of style ]
let print : [< t ] -> unit = function
  | `Begin_style `Bold -> ()
  | `Begin_style `Italic -> ()

The infinite loop happens somewhere in a call to Parmatch.check_unused.
Bisecting the issue points to commit 2250fd8