Suggest -XDeriveDataTypeable with "newtype" too

I'm forwarding wishlist bug #499216 from Debian's BTS.

---

If I try to compile or load this program, ghc complains that I need to pass -XDeriveDataTypeable to enable the derivation of Typeable instances:

------ begin Test.hs --------
module Test where

import Data.Typeable

data List a = List [a] deriving(Typeable)
-----------------------------

Test.hs:5:0:
    Can't make a derived instance of `Typeable (List a)'
      (You need -XDeriveDataTypeable to derive an instance for this class)
    In the data type declaration for `List'

If I use "newtype" and I have GeneralizedNewtypeDeriving turned on, however, I get this error:

/tmp/Test.hs:5:0:
    Can't make a derived instance of `Typeable (List a)'
      (even with cunning newtype deriving:)
    In the newtype declaration for `List'

It would be nice if ghc suggested DeriveDataTypeable here too. JFTR, it does suggest this if newtype deriving is off:

/tmp/Test.hs:5:0:
    Can't make a derived instance of `Typeable (List a)'
      (You need -XDeriveDataTypeable to derive an instance for this class
       Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension)
    In the newtype declaration for `List'

Trac metadata

Trac field	Value
Version	6.8.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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Operating system	Unknown
Architecture	Unknown