[GHC] #3850: EmptyDataDecls and type context

[GHC]

#3850: EmptyDataDecls and type context
---------------------------------+------------------------------------------
    Reporter:  Paczesiowa        |       Owner:                           
        Type:  bug               |      Status:  new                      
    Priority:  normal            |   Component:  Compiler                 
     Version:  6.12.1            |    Keywords:  emptydatadecls           
          Os:  Unknown/Multiple  |    Testcase:                           
Architecture:  Unknown/Multiple  |     Failure:  GHC rejects valid program
---------------------------------+------------------------------------------
 the following code fails to parse correctly or typecheck under 6.12.1


 {{{
 {-# LANGUAGE EmptyDataDecls #-}
 data Show a => Foo a
 }}}


 error:


 {{{
     No context is allowed on a GADT-style data declaration
     (You can put a context on each contructor, though.)
 }}}


 it works with 6.10.

 this is the reason of HList failure on 6.12.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3850>
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