GHC Panic with TypeInType / levity polymorphism

{-# Language TypeInType #-}
{-# Language TypeOperators, DataKinds, PolyKinds #-}

import Data.Kind
import GHC.Types (TYPE)

data Exp :: [TYPE rep] -> TYPE rep -> Type where
  Lam :: Exp (a:xs) b -> Exp xs (a -> b)

gives

$ ghci -ignore-dot-ghci hs/126-bug.hs
GHCi, version 8.3.20171122: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( hs/126-bug.hs, interpreted )
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.3.20171122 for x86_64-unknown-linux):
	zonkTcTyVarToVar
  'LiftedRep
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1147:37 in ghc:Outputable
        pprPanic, called at compiler/typecheck/TcType.hs:1640:17 in ghc:TcType

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

caused by TypeInType. Removing the first line, we get an error

GHCi, version 8.3.20171122: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( hs/126-bug.hs, interpreted )

hs/126-bug.hs:7:19: error:
    • Expected kind ‘GHC.Types.RuntimeRep’, but ‘rep’ has kind ‘*’
    • In the first argument of ‘TYPE’, namely ‘rep’
      In the kind ‘[TYPE rep] -> TYPE rep -> Type’
  |
7 | data Exp :: [TYPE rep] -> TYPE rep -> Type where
  |                   ^^^

hs/126-bug.hs:7:32: error:
    • Expected kind ‘GHC.Types.RuntimeRep’, but ‘rep’ has kind ‘*’
    • In the first argument of ‘TYPE’, namely ‘rep’
      In the kind ‘[TYPE rep] -> TYPE rep -> Type’
  |
7 | data Exp :: [TYPE rep] -> TYPE rep -> Type where
  |                                ^^^
Failed, no modules loaded.
Prelude> 

Trac metadata

Trac field	Value
Version	8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import TypeInType labels

Simpler example

data Exp :: TYPE rep -> Type where
  Lam :: Exp (a -> b)

can be fixed by explicitly quantifying over rep

data Exp :: forall rep. TYPE rep -> Type where -- ...
--
data Exp :: forall (rep::RuntimeRep). TYPE rep -> Type where -- ...

mentioned in issue #14563 (closed)

Trac metadata

Trac field	Value
Related	- → #14563 (closed)

mentioned in commit 8361b2c5

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed
Test case	- → polykinds/T14555

added representation polymorphism label

added Pnormal label