[GHC] #15712: GHC panic with -XDerivingVia

[GHC]

#15712: GHC panic with -XDerivingVia
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:  DerivingVia
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 This is without the https://hackage.haskell.org/package/kan-
 extensions-5.2/docs/Control-Monad-Codensity.html dependency

 {{{#!hs
 {-# Language RankNTypes    #-}
 {-# Language DerivingVia   #-}
 {-# Language DeriveFunctor #-}

 -- import Control.Monad.Codensity
 import Data.Kind

 newtype Codensity f a = Codensity (forall xx. (a -> f xx) -> f xx)
   deriving
     Functor

 newtype GEndo m a = GEndo (m a -> m a)

 newtype LogicT m a = LogicT (forall xx. (a -> (m xx -> m xx)) -> (m xx ->
 m xx))
   deriving
     (Functor)
   via
     (Codensity GEndo)
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15712#comment:2>
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