[GHC] #7156: "Pattern match on GADT" error for non-GADT

[GHC]

#7156: "Pattern match on GADT" error for non-GADT
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    Reporter:  ryani                                           |       Owner:                  
        Type:  bug                                             |      Status:  new             
    Priority:  normal                                          |   Milestone:                  
   Component:  Compiler (Type checker)                         |     Version:  7.0.4           
    Keywords:  GADTs, TypeFamilies, ExistentialQuantification  |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple                                |     Failure:  None/Unknown    
  Difficulty:  Unknown                                         |    Testcase:                  
   Blockedby:                                                  |    Blocking:                  
     Related:                                                  |  
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Comment(by ryani):

 Interesting workaround I just found...

 {{{
 {-# LANGUAGE TypeFamilies, ExistentialQuantification #-}
 module NonGADT where

 class (a ~ ()) => C a
 instance C ()

 data T a = (C a) => T
 f :: T a -> a
 f T = ()
 }}}

 typechecks successfully under GHC7.4.1.  It seems to work identically to
 GADTs, including inferring the type of {{{ T }}} as {{{ T () }}}.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7156#comment:4>
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