no implicit group decoder for case class with more than 1 field with non-standard fields

[rubenpieters]

if we have the following case class:

case class CC4(str: String, dt: LocalDate)

Then there is no GroupDecoder[CC4]: implicitly[GroupDecoder[CC4]] does not compile.
Which results in it not being usable in for example a Regex[(A, CC4)], but Regex[CC4] works fine.

Interestingly, this only applies if the case class has more than 1 field

here is a gist with some examples: https://gist.github.com/rubenpieters/3227060a0ab55c48ea6ac899f70a115e