[GHC] #9072: RankNTypes composition now requires ImpredicativeTypes

[GHC]

#9072: RankNTypes composition now requires ImpredicativeTypes
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       Reporter:  gelisam            |             Owner:
           Type:  bug                |            Status:  new
       Priority:  normal             |         Milestone:
      Component:  Compiler (Type     |           Version:  7.8.2
  checker)                           |  Operating System:  Unknown/Multiple
       Keywords:                     |   Type of failure:  GHC rejects
   Architecture:  Unknown/Multiple   |  valid program
     Difficulty:  Unknown            |         Test Case:
     Blocked By:                     |          Blocking:
Related Tickets:                     |
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 {{{
 {-# LANGUAGE RankNTypes #-}

 instantiateId :: (forall a. a -> a) -> Int -> Int
 instantiateId f = f

 passes :: [Int] -> [Int]
 passes = map (instantiateId id)

 -- Couldn't match type ‘a0 -> a0’ with ‘forall a. a -> a’
 fails :: [Int] -> [Int]
 fails = (map . instantiateId) id
 }}}

 works with ghc 7.6,
 fails with ghc 7.8,
 works with ghc 7.8 + `ImpredicativeTypes`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9072>
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