Type unsoundness in Haskell

unsoundness.hs

{-# LANGUAGE GeneralizedNewtypeDeriving, TypeFamilies #-}

-- related to http://hackage.haskell.org/trac/ghc/ticket/1496

type family F a
type instance F Int = Int
type instance F X   = Int -> Int

class Fable a where
    fify :: a -> F a

instance Fable Int where
    fify = id

newtype X = X Int
    deriving Fable

main = print (fify (X 0) 0)