[GHC] #2612: deriving Data.Typeable on phantom newtypes does not report that DeriveDataTypeable is needed

[GHC]

#2612: deriving Data.Typeable on phantom newtypes does not report that
DeriveDataTypeable is needed
-----------------------+----------------------------------------------------
    Reporter:  guest   |       Owner:          
        Type:  bug     |      Status:  new     
    Priority:  normal  |   Component:  Compiler
     Version:  6.9     |    Severity:  normal  
    Keywords:          |    Testcase:          
Architecture:  x86     |          Os:  MacOS X 
-----------------------+----------------------------------------------------
 newtype Foo x = Foo deriving (Typeable)

 reports the rather cryptic error message:
     Can't make a derived instance of `Typeable (Foo x)'
       (even with cunning newtype deriving:)
     In the newtype declaration for `Foo'

 When newtype is replaced with data, this much more useful error message is
 given:
     Can't make a derived instance of `Typeable (Foo x)'
       (You need -XDeriveDataTypeable to derive an instance for this class)
     In the data type declaration for `Foo'

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2612>
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