[GHC] #7861: deferred type error with rankNTypes

[GHC]

#7861: deferred type error with rankNTypes
-------------------------------+--------------------------------------------
    Reporter:  guest           |       Owner:                    
        Type:  bug             |      Status:  new               
    Priority:  normal          |   Milestone:                    
   Component:  Compiler        |     Version:  7.6.2             
    Keywords:                  |          Os:  Linux             
Architecture:  x86_64 (amd64)  |     Failure:  Compile-time crash
  Difficulty:  Unknown         |    Testcase:                    
   Blockedby:                  |    Blocking:                    
     Related:                  |  
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Comment(by monoidal):

 A bit smaller version crashing 7.6:

 {{{
 {-# LANGUAGE RankNTypes #-}

 type A a = forall b. a

 doA :: A a -> [a]
 doA = undefined

 f :: A a -> a
 f = doA

 main = return ()
 }}}

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7861#comment:3>
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