[GHC] #15712: GHC panic with -XDerivingVia

[GHC]

#15712: GHC panic with -XDerivingVia
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           Keywords:  DerivingVia    |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I slipped up and typed `Codensity GEndo` and now `Codensity (GEndo m)`,
 resulting in a GHC panic

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

 import Control.Monad.Codensity
 import Data.Kind

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

 newtype LogicT m a = LogicT { runLogicT :: forall xx. (a -> (m xx -> m
 xx)) -> (m xx -> m xx) }
   deriving
     (Functor, Applicative, Monad)
   via
     (Codensity GEndo)
 }}}

 {{{
 $ ghci -ignore-dot-ghci 476.hs
 GHCi, version 8.7.20180828: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( 476.hs, interpreted )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.7.20180828 for x86_64-unknown-linux):
         ASSERT failed!
   Type-correct unfilled coercion hole {co_a2aG}
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
 ghc:Outputable
         pprPanic, called at compiler/utils/Outputable.hs:1219:5 in
 ghc:Outputable
         assertPprPanic, called at compiler/typecheck/TcHsSyn.hs:1716:99 in
 ghc:TcHsSyn

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

 >
 }}}

 Deriving via `Codensity (GEndo m)` works as expected.

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