{-# Language RankNTypes       #-}
{-# Language TypeApplications #-}
{-# Language PolyKinds        #-}
{-# Language KindSignatures   #-}
{-# Language TypeFamilies     #-}

data family   Pair :: forall a. forall b. *
data instance Pair @a @b = MkPair a b

$ ghci -ignore-dot-ghci 593_bug.hs
GHCi, version 8.7.20181017: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( 593_bug.hs, interpreted )
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.7.20181017 for x86_64-unknown-linux):
        ASSERT failed!
  kind axiom D:R:Pairabab0 ::
         forall a b (a :: a) (b :: b).
           Pair = R:Pairabab -- Defined at 593_bug.hs:8:15
  forall k k (a :: k) (b :: k). *
  forall a b -> a -> b -> *
  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/FamInst.hs:158:61 in ghc:FamInst

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

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