Haskell: Instance definitions for type families

Question

Lets say we have the following code:

class C t where
  g :: t

instance C Int where
  g = 42

Simple. We can also define functions on Int, like

Answer 1

The following file compiles here:

{-# LANGUAGE GeneralizedNewtypeDeriving, TypeFamilies #-}

class C a where g :: a
type family X
type instance X = Int
newtype NewX = NewX X deriving Num
instance C NewX where g = 43