[GHC] #15796: Core Lint error

[GHC]

#15796: Core Lint error
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 This gives a Core Lint error

 {{{#!hs
 {-# Language QuantifiedConstraints #-}
 {-# Language TypeApplications      #-}
 {-# Language TypeOperators         #-}
 {-# Language PolyKinds             #-}
 {-# Language FlexibleInstances     #-}
 {-# Language DataKinds             #-}
 {-# Language TypeFamilies          #-}
 {-# Language MultiParamTypeClasses #-}
 {-# Language ConstraintKinds       #-}
 {-# Language UndecidableInstances  #-}
 {-# Language GADTs                 #-}

 {-# Options_GHC -dcore-lint #-}

 import Data.Kind

 type Cat ob = ob -> ob -> Type

 class Ríki (obj :: Type) where
   type (-->) :: Cat obj

 class Varpi (f :: dom -> cod)

 newtype
   (··>) :: Cat (a -> b) where
   Natu :: Varpi f
        => (forall xx. f xx --> f' xx)
        -> (f ··> f')

 instance
      Ríki cod
   => -------------
      Ríki (dom -> cod)
   where

   type (-->) = (··>) @dom @cod
 }}}

 {{{
 $ ghci -ignore-dot-ghci 568_bug.hs
 GHCi, version 8.7.20181017: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( 568_bug.hs, interpreted )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.7.20181017 for x86_64-unknown-linux):
         Core Lint error
   <no location info>: warning:
       In the type ‘(··>)’
       Found TcTyCon: ··>[tc]
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
 ghc:Outputable
         pprPanic, called at compiler/typecheck/FamInst.hs:171:31 in
 ghc:FamInst

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

 > :q
 Leaving GHCi.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15796>
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