[GHC] #11428: ImpredicativeTypes causes GHC panic with 8.0.1-rc1

[GHC]

#11428: ImpredicativeTypes causes GHC panic with 8.0.1-rc1
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  8.0.1-rc1
  checker)                           |             Keywords:
      Resolution:  fixed             |  ImpredicativeTypes,
                                     |  TypeApplications
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by bgamari):

 * status:  new => closed
 * resolution:   => fixed


Comment:

 Indeed this now fails with the error
 {{{
 T11428.hs:12:13: error:
     • Couldn't match expected type ‘forall a. a -> a’
                   with actual type ‘Pipe o2 r0 -> Pipe o2 r0’
     • When checking that: Pipe o2 r0 -> Pipe o2 r0
         is more polymorphic than: forall a. a -> a
       The lambda expression ‘\ o' p' -> HaveOutput p' o'’
       has two arguments,
       but its type ‘o2 -> forall a. a -> a’ has only one
       In the second argument of ‘maybe’, namely
         ‘(\ o' p' -> HaveOutput p' o')’
     • Relevant bindings include
         f :: o1 -> Maybe o2 (bound at T11428.hs:11:16)
         mapOutputMaybe :: (o1 -> Maybe o2) -> Pipe o1 r -> Pipe o2 r
           (bound at T11428.hs:11:1)
 }}}

 When compiled with `ImpredicativeTypes`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11428#comment:3>
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