{-# language TypeFamilies, UndecidableInstances #-}

import GHC.Generics

data TNothing = TNothing
data TJust a = TJust a

type family Merge a b where
    Merge TNothing x = x
    Merge (TJust x) TNothing = TJust x

class Something tag f where
    something :: f p -> tag

-- Apparently Merge can't be used on the right side, like this:
-- instance (Something tagF f, Something tagG g) => Something (Merge tagF tagG) (f :*: g) where
instance (Something tagF f, Something tagG g, tag ~ Merge tagF tagG) => Something tag (f :*: g) where
    something = undefined