GHC Panic with QuantifiedConstraints

From branch wip/T2893

{-# Language QuantifiedConstraints #-}
{-# Language StandaloneDeriving #-}
{-# Language DataKinds #-}
{-# Language TypeOperators #-}
{-# Language GADTs #-}
{-# Language KindSignatures #-}
{-# Language FlexibleInstances #-}
{-# Language UndecidableInstances #-}
{-# Language MultiParamTypeClasses #-}
{-# Language RankNTypes #-}
{-# Language ConstraintKinds #-}

import Data.Kind

data D c where
  D :: c => D c

newtype a :- b = S (a => D b)

class C1 a b
class C2 a b
instance C1 a b => C2 a b

class    (forall xx. f xx) => Limit f
instance (forall xx. f xx) => Limit f

-- impl :: Limit (C1 a) :- Limit (C2 a)
-- impl = S D

infixr 5 :<
data Sig a = N a | a :< Sig a

data AST :: (Sig Type -> Type) -> (Sig Type -> Type) where
  Sym  :: dom a -> AST dom a
  (:$) :: AST dom (xx :< a) -> AST dom (N xx) -> AST dom a

deriving instance (forall xx. Show (dom xx)) => Show (AST dom a)

data Arith a where
  Plus :: Arith (Int :< Int :< N Int)

deriving instance Show (Arith a)

loading this program and evaluating Sym Plus works fine:

$ ghc-stage2 --interactive hs/175-bug.hs
GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( hs/175-bug.hs, interpreted )
Ok, one module loaded.
*Main> Sym Plus
Sym Plus
*Main> 

but we uncomment impl we get a panic!

GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( hs/175-bug.hs, interpreted )
Ok, one module loaded.
*Main> Sym Plus
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.5.20180128 for x86_64-unknown-linux):
	nameModule
  system df_a2VB
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable
        pprPanic, called at compiler/basicTypes/Name.hs:241:3 in ghc:Name

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

*Main> 

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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