[GHC] #13343: Levity polymorphism-related GHC panic: expectJust zonkTcTyVarToVar

[GHC]

#13343: Levity polymorphism-related GHC panic: expectJust zonkTcTyVarToVar
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
  LevityPolymorphism                 |
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Reproducible with GHC 8.0.2 and HEAD:

 {{{#!hs
 {-# 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
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13343>
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