I think that similar issues have been reported before, and seem to arise
out of a wrong interaction between ppx_deriving and ppx_type_conv:

  https://github.com/ocaml-ppx/ppx_deriving/issues/149
  https://github.com/janestreet/ppx_type_conv/issues/9

(Note: Jérémie Dimino, which has been handling most of the ppx_type_conv
maintenance, is currently in well-deserved holidays, so the safest bet for
the next couple months may indeed be to pin back to older versions that
work for you.)

On Fri, Oct 6, 2017 at 10:29 AM, Emilio Jesús Gallego Arias <e@x80.org>
wrote:

> Dear Christoph,
>
> Christoph Höger <christoph.hoeger@celeraone.com> writes:
>
> > Does anyone know a workaround?
>
> I had a similar problem [but not with visitors] that I solved by pinning
> ppx_deriving to the 4.1 version. I guess you lose little by not trying.
>
> E.
>
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