[GHC] #9892: Decomposition of AppTy broken

[GHC]

#9892: Decomposition of AppTy broken
-------------------------------------+-------------------------------------
       Reporter:  simonpj            |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 This program (taken from `lens`) wrongly fails in GHC's HEAD
 {{{
   {-# LANGUAGE UndecidableInstances #-}

   import Control.Applicative
   import Control.Category
   import Prelude hiding ((.),id)

   newtype FocusingPlus w k s a = FocusingPlus { unfocusingPlus :: k (s, w)
 a }

   instance Functor (k (s, w)) => Functor (FocusingPlus w k s) where
     fmap f (FocusingPlus as) = FocusingPlus (fmap f as)

   instance Applicative (k (s, w)) => Applicative (FocusingPlus w k s)
 where
     pure = FocusingPlus . pure
     FocusingPlus kf <*> FocusingPlus ka = FocusingPlus (kf <*> ka)
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9892>
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