Hello, Given the following program: ----------------------------------------------------------- {-# LANGUAGE DeriveDataTypeable, TypeFamilies #-} import Data.Typeable class C a where data T a :: * data MyType1 = MyType1 deriving Typeable data MyType2 = MyType2 deriving Typeable instance C MyType1 where data T MyType1 = A1 deriving (Typeable) instance C MyType2 where data T MyType2 = A2 deriving (Typeable) ----------------------------------------------------------- I get the following unexpected error: TF_Data.hs:12:34: Duplicate instance declarations: instance Typeable1 T -- Defined at TF_Data.hs:12:34 instance Typeable1 T -- Defined at TF_Data.hs:15:34 When looking at the output of -ddump-deriv I see that the following instance is generated: instance Typeable1 T where ... I would have expected that the following instances were generated instead: instance Typeable1 (T MyType1) where ... instance Typeable1 (T MyType2) where ... Is this a bug in GHC? Thanks, Bas