{-# Language    RankNTypes          #-}
{-# Language    TypeFamilies        #-}
{-# Language    ScopedTypeVariables #-}
{-# Language    TypeApplications    #-}
{-# Language    DataKinds           #-}
{-# Language    PolyKinds           #-}
{-# Language    TypeOperators       #-}
{-# Language    GADTs               #-}

{-# Options_GHC -dcore-lint #-}

import Data.Kind

class Ríki (obj :: Type) where
  type Obj :: obj -> Constraint
  type Obj = Bæ @obj

class    Bæ    (a :: k)
instance Bæ @k (a :: k)

data
  EQ :: forall ob. ob -> ob -> Type where
  EQ :: EQ a a

instance
  Ríki (EQ @ob)

$ ghci -dcore-lint 568_bug.hs
GHCi, version 8.7.20181017: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( 568_bug.hs, interpreted )
WARNING: file compiler/types/TyCoRep.hs, line 2567
  in_scope InScope {ob_a1zs co_a1zt}
  tenv []
  cenv [a1zt :-> co_a1zt]
  tys [Bæ]
  cos []
  needInScope [a1zn :-> co_a1zn, a1zs :-> ob_a1zs]
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 ‘Bæ’
      Unfilled coercion hole: {co_a1zn}
  <no location info>: warning:
      In the type ‘Bæ’
      co_a1zn :: (ob_a1zs -> ob_a1zs -> *) ~# *
      [LclId[CoVarId]] is out of scope
  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

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