[GHC] #5007: "deriving" seems to ignore class context for a type family

[GHC]

#5007: "deriving" seems to ignore class context for a type family
---------------------------------+------------------------------------------
    Reporter:  jkff              |       Owner:                                                          
        Type:  bug               |      Status:  new                                                     
    Priority:  normal            |   Component:  Compiler (Type checker)                                 
     Version:  7.0.2             |    Keywords:  type families, datatype contexts, type classes, deriving
    Testcase:                    |   Blockedby:                                                          
          Os:  Unknown/Multiple  |    Blocking:                                                          
Architecture:  Unknown/Multiple  |     Failure:  GHC rejects valid program                               
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 {{{
 class Foo a where
   data Bar a :: *

 class (Show (Bar a)) => Qux a
 data (Qux a) => Xyzzy a = Xyzzy (Bar a) deriving Show
 }}}

 Here I get the following error:
 {{{
     No instance for (Show (Bar a))
       arising from the 'deriving' clause of a data type declaration
     Possible fix:
       add an instance declaration for (Show (Bar a))
       or use a standalone 'deriving instance' declaration,
            so you can specify the instance context yourself
     When deriving the instance for (Show (Xyzzy a))
 }}}

 Curiously, I get the same error even if I add ", Show (Bar a)" to the
 context of "Xyzzy".

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5007>
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