[GHC] #7888: Impredicativity flag needed more often

[GHC]

#7888: Impredicativity flag needed more often
----------------------------------------------+-----------------------------
Reporter:  monoidal                           |          Owner:                         
    Type:  bug                                |         Status:  new                    
Priority:  normal                             |      Component:  Compiler (Type checker)
 Version:  7.6.3                              |       Keywords:                         
      Os:  Unknown/Multiple                   |   Architecture:  Unknown/Multiple       
 Failure:  Incorrect warning at compile-time  |      Blockedby:                         
Blocking:                                     |        Related:                         
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 I don't know if this is a bug or a deliberate change, but HEAD requires
 -XImpredicativeTypes for this program while 7.6.3 does not.

 {{{
 {-# LANGUAGE RankNTypes #-}

 f :: (forall a. a) -> b
 f = undefined
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7888>
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