Levity polymorphism-related GHC panic: expectJust zonkTcTyVarToVar

Reproducible with GHC 8.0.2 and HEAD:

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeInType #-}
module Bug where

import GHC.Exts

type Bad = forall (v1 :: RuntimeRep) (a1 :: TYPE v). a1

[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.1.20170223 for x86_64-unknown-linux):
        expectJust zonkTcTyVarToVar
CallStack (from HasCallStack):
  error, called at compiler/utils/Maybes.hs:53:27 in ghc:Maybes
  expectJust, called at compiler/typecheck/TcType.hs:1576:21 in ghc:TcType

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture