[GHC] #15625: GHC panic, with QuantifiedConstraints

[GHC]

#15625: GHC panic, with QuantifiedConstraints
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1-beta1
           Keywords:                 |  Operating System:  Unknown/Multiple
  QuantifiedConstraints              |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I got a GHC Panic (I made some minor changes to GHC so it may have been
 added by me) but I think it's caused by the quality constraint

 {{{
 $ ~/code/latestghc/inplace/bin/ghc-stage2 --interactive -ignore-dot-ghci
 ~/hs/390.hs
 GHCi, version 8.7.20180828: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( /home/baldur/hs/390.hs, interpreted
 )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.7.20180828 for x86_64-unknown-linux):
         ASSERT failed!
   co_a2DG
   df_a2DS @ Any
   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/coreSyn/CoreSubst.hs:189:49 in
 ghc:CoreSubst

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

 >
 }}}

 given this code

 {{{#!hs
 {-# Language RankNTypes, TypeInType, DataKinds, PolyKinds, TypeOperators,
 GADTs, FlexibleInstances, MultiParamTypeClasses, ConstraintKinds, CPP,
 UndecidableSuperClasses, QuantifiedConstraints, FlexibleContexts #-}

 import Data.Kind

 type Cat ob = ob -> ob -> Type

 data KLEISLI (m :: Type -> Type) :: Cat (KL_kind m) where
   MkKLEISLI :: (a -> m b) -> KLEISLI(m) (KL a) (KL b)

 data KL_kind (m :: Type -> Type) = KL Type

 class    (a ~ KL xx) => AsKL a xx
 instance (a ~ KL xx) => AsKL a xx

 ekki__ :: Monad m => (forall xx. AsKL a xx) => KLEISLI m a a
 ekki__ = MkKLEISLI undefined
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15625>
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