{-# Language RankNTypes   #-}
{-# Language DataKinds    #-}
{-# Language PolyKinds    #-}
{-# Language GADTs        #-}
{-# Language TypeFamilies #-}

import Data.Kind

data Var where
  Op :: Var
  Id :: Var

-- type family
--   Flip (var :: Var) = (res :: Var) | res -> var where
--   Flip Op = Id
--   Flip Id = Op

type Varianced = (forall (var :: Var). Type)

-- data A :: Varianced where
--   MkA :: Int -> A @Id

-- data OpA :: Varianced where
--   MkOpA :: Int -> OpA @Op

-- data Exp :: Varianced -> Varianced where
--   X :: forall (ext::Varianced) (var :: Var). Exp ext @var
  -- OpExp1 :: forall (var :: Varianced). var @op -> Exp var @(Flip op)

data family   Parser :: Varianced
data instance Parser = P

$ ~/code/unmodifiedghc/inplace/bin/ghc-stage2 --interactive -ignore-dot-ghci 656.hs
GHCi, version 8.7.20181029: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( 656.hs, interpreted )
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.7.20181029 for x86_64-unknown-linux):
        ASSERT failed!
  kind axiom D:R:Parservar0 ::
         Parser = R:Parservar -- Defined at 656.hs:31:15
  forall (var :: Var). *
  Var -> *
  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

>