Deriving Typeable for data families

Bas points out that deriving Typeable for data families isn't right at all.

{-# LANGUAGE DeriveDataTypeable, TypeFamilies #-}

import Data.Typeable

class C a where
    data T a :: *

instance C Int where
    data T Int = A1 deriving (Typeable)

instance C Bool where
    data T Bool = A2 deriving (Typeable)

gives this 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

The real problem is that we should have only one instance for Typeable T. We should only allow

deriving instance Typeable1 T

and prohibit deriving Typeable on data instance declarations.

Trac metadata

Trac field	Value
Version	7.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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