Note [Bindings for Generalised Newtype Deriving]

Consider
  class Eq a => C a where
     f :: a -> a
  newtype N a = MkN [a] deriving( C )
  instance Eq (N a) where ...

The 'deriving C' clause generates, in effect
  instance (C [a], Eq a) => C (N a) where
     f = coerce (f :: [a] -> [a])

This generates a cast for each method, but allows the superclasses to
be worked out in the usual way.  In this case the superclass (Eq (N
a)) will be solved by the explicit Eq (N a) instance.  We do *not*
create the superclasses by casting the superclass dictionaries for the
representation type.

See the paper "Safe zero-cost coercions for Haskell".