LabelledGeneric derivation fails sometimes depending on where the coproduct is defined

[alexarchambault]

• First example described in Enabled sum-types tests argonaut-io/argonaut#142
• Second example before and after the last commit in https://github.com/alexarchambault/argonaut/tree/bug/shapeless-labelledgeneric (which uses https://github.com/milessabin/shapeless/tree/topic/serialization)

[milessabin]

This will be a consequence of SI-7755.

I'm fairly sure I've done the best that's possible here (search for mentions of this issue in generic.scala) but if you have any additional insights they would be most welcome.

[alexarchambault]

Thks, closing this for now then...