Foldable, Functor, and Traversable deriving handle phantom types badly

Suppose we have

data Phantom a = Z | S (Phantom a)

We'd like to get something like

foldMap _ _ = mempty
fmap = coerce
traverse _ m = pure (coerce m)

But instead we actually pattern match all the way down! Basically, we want to treat "has a phantom role" and "does not occur" similarly.

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