These shouldn't be hard to implement - all we need is a conversion to some concrete representation. For example, for LocalDate we can use Cogen[Long].contramap(_.toEpochDay).

As for the motivation, cats makes extensive use of Cogens in cats-laws.

https://github.com/alibaba/dragonwell8

Currently, our specs only test if a service is properly registered on classpath ("proper service is registered on classpath"). It's easy to check, because we're testing on classpath only (due to Groovy 2). However, we should find some way to also test if a service is properly registered on modulepath (and generally test everything on modulepath too - should be possible with Groovy 3).