deepseq: instance NFData (a -> b)

[Niklas Hambüchen]

I wound find it interesting to learn what exactly will break when this
instance is removed / what feature motivated the addition of the einstance.

I'm taking a guess here, maybe it's Generics based deriving?

    data D = D { something :: Int, somethingElse :: Int -> String }
      deriving Generic

    deriving instance NFData D

Is it this?