[GHC] #13929: GHC panic with levity polymorphism

[GHC]

#13929: GHC panic with levity polymorphism
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           Reporter:  vagarenko       |             Owner:  (none)
               Type:  bug             |            Status:  new
           Priority:  high            |         Milestone:  8.2.1
          Component:  Compiler        |           Version:  8.2.1-rc2
           Keywords:                  |  Operating System:  Windows
       Architecture:  x86_64 (amd64)  |   Type of failure:  None/Unknown
          Test Case:                  |        Blocked By:
           Blocking:                  |   Related Tickets:
Differential Rev(s):                  |         Wiki Page:
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 I'm using GHC version 8.2.0.20170507

 This code fails to compile

 {{{#!hs
 {-# LANGUAGE MagicHash #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE DefaultSignatures #-}
 {-# LANGUAGE UnboxedTuples #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE UndecidableInstances #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE AllowAmbiguousTypes #-}

 module Main where

 import GHC.Exts
 import Data.Kind
 import GHC.Generics

 class GUnbox (f :: Type -> Type) (r :: RuntimeRep) where
     type GUnboxed f r :: TYPE r
     gunbox :: f p -> GUnboxed f r

 instance (GUnbox f rf, GUnbox g rg) => GUnbox (f :*: g) ('TupleRep '[rf,
 rg]) where
     type GUnboxed (f :*: g) ('TupleRep '[rf, rg]) = (# GUnboxed f rf,
 GUnboxed g rg #)
     -- if I remove implementation of `gunbox` it compiles successfully
     gunbox (x :*: y) = (# gunbox x, gunbox y #)

 main :: IO ()
 main = pure ()
 }}}

 with message:


 {{{
 [1 of 1] Compiling Main             ( Main.hs, .stack-
 work\dist\f42fcbca\build\Main.o )
 ghc.EXE: panic! (the 'impossible' happened)
   (GHC version 8.2.0.20170507 for x86_64-unknown-mingw32):
         isUnliftedType
   GUnboxed g_a21y rg_a21z :: TYPE rg_a21z
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler\utils\Outputable.hs:1134:58 in ghc:Outputable
         callStackDoc, called at compiler\utils\Outputable.hs:1138:37 in
 ghc:Outputable
         pprPanic, called at compiler\types\Type.hs:1954:10 in ghc:Type

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

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13929>
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