[GHC] #6147: GeneralizedNewtypeDeriving should fail with data families

[GHC]

#6147: GeneralizedNewtypeDeriving should fail with data families
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 Reporter:  rl                           |          Owner:                  
     Type:  bug                          |         Status:  new             
 Priority:  normal                       |      Component:  Compiler        
  Version:  7.2.1                        |       Keywords:                  
       Os:  Unknown/Multiple             |   Architecture:  Unknown/Multiple
  Failure:  GHC accepts invalid program  |       Testcase:                  
Blockedby:                               |       Blocking:                  
  Related:                               |  
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 Here is an example:

 {{{
 {-# LANGUAGE GeneralizedNewtypeDeriving, TypeFamilies #-}
 module Foo where

 data family T a
 data instance T Int = T_Int Int

 class C a where
   foo :: a -> T a

 instance C Int where
   foo = T_Int

 newtype Foo = Foo Int deriving(C)
 }}}

 The `deriving(C)` clause on `Foo` should fail but instead it derives an
 instance which can't possible be correct since there is no corresponding
 `data instance T Foo`. This is closely related to #2721 (where newtype
 deriving was disabled in the presence of associated types) but much more
 difficult to test for. Probably also related to #1496.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6147>
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