Reiner Pope reports that the following code

{-# LANGUAGE DeriveGeneric, FlexibleInstances, StandaloneDeriving #-}
import GHC.Generics
data T a = T a
deriving instance Generic (T Bool)

generates the following:

instance Generic (T Bool) where ...
type instance Rep (T a) = ...

This is wrong. The type instance should probably be for Rep (T Bool). Or perhaps we should refuse to derive Generic for instantiated types altogether.