Note [Newtype-deriving trickiness]

Consider (#12768):
  class C a where { op :: D a => a -> a }

  instance C a  => C [a] where { op = opList }

  opList :: (C a, D [a]) => [a] -> [a]
  opList = ...

Now suppose we try GND on this:
  newtype N a = MkN [a] deriving( C )

The GND is expecting to get an implementation of op for N by
coercing opList, thus:

  instance C a => C (N a) where { op = opN }

  opN :: (C a, D (N a)) => N a -> N a
  opN = coerce @([a]   -> [a])
               @([N a] -> [N a]
               opList :: D (N a) => [N a] -> [N a]

But there is no reason to suppose that (D [a]) and (D (N a))
are inter-coercible; these instances might completely different.
So GHC rightly rejects this code.