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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