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I'm using GHC version 8.2.0.20170507

This code fails to compile

{-# 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

Trac metadata

Trac field	Value
Version	8.2.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Windows
Architecture	x86_64 (amd64)