[GHC] #7873: A poly-kinded newtype existential crisis

[GHC]

#7873: A poly-kinded newtype existential crisis
--------------------------------------+-------------------------------------
Reporter:  ekmett                     |          Owner:                  
    Type:  bug                        |         Status:  new             
Priority:  normal                     |      Component:  Compiler        
 Version:  7.6.3                      |       Keywords:                  
      Os:  Unknown/Multiple           |   Architecture:  Unknown/Multiple
 Failure:  GHC rejects valid program  |      Blockedby:                  
Blocking:                             |        Related:                  
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 The following code worked on older GHCs and on other compilers clear back
 to Hugs, but based on reports I'm getting from a half-dozen users appears
 to broken on GHC 7.6.1+ and in GHC HEAD.

 {{{
 class Reifies s a | s -> a where
   reflect :: proxy s -> a

 data Proxy a = Proxy

 newtype Magic a r = Magic (forall s. Reifies s a => Proxy s -> r)
 }}}

 {{{
 fast/Data/Reflection.hs:92:21:
     A newtype constructor cannot have an existential context,
       but `Magic' does
     In the definition of data constructor `Magic'
 }}}

 There doesn't appear to be anything existential going on there, but that
 said, this issue does only occur when PolyKinds are turned on, so perhaps
 the issue is with the desugaring into PolyKinds?

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7873>
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