panic

[1 of 1] Compiling Repro            ( ScopedEffectsRepro.hs, ScopedEffectsRepro.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.6.2 for x86_64-unknown-linux):
	StgCmmEnv: variable not found
  $dIMonad_a1lH
  local binds for:
  $tc'VarE
  $tcE
  $tcIMonad
  $trModule
  return
  >>=
  $tc'VarE1_r1oq
  $tc'VarE2_r1pa
  $krep_r1pb
  $krep1_r1pc
  $krep2_r1pd
  $krep3_r1pe
  $krep4_r1pf
  $tcE1_r1pg
  $tcE2_r1ph
  $tcIMonad1_r1pi
  $tcIMonad2_r1pj
  $krep5_r1pk
  $krep6_r1pl
  $krep7_r1pm
  $trModule1_r1pn
  $trModule2_r1po
  $trModule3_r1pp
  $trModule4_r1pq
  $krep8_r1pr
  $krep9_r1ps
  sat_s1ro
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable
        pprPanic, called at compiler/codeGen/StgCmmEnv.hs:149:9 in ghc:StgCmmEnv

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

ScopedEffectsRepro.hs

{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE InstanceSigs #-}
{-# LANGUAGE RebindableSyntax #-}

{-# OPTIONS_GHC -Wall -Wno-missing-signatures -Wno-unticked-promoted-constructors
               -Wno-name-shadowing -fwarn-partial-type-signatures -Wno-partial-type-signatures #-}
module Repro(main) where

import Prelude hiding (Monad(..))
import Control.Applicative

data E (a :: * -> *) (n :: *) where
  VarE :: a n -> E a n

instance IMonad E where
  return :: a n -> E a n
  return = VarE

  (>>=) :: E a n -> (forall n . a n -> E b n) -> E b n
  VarE x >>= f = f x

class IMonad (m :: (* -> *) -> (* -> *)) where
  return :: forall a n . a n -> m a n
  (>>=) :: m a n -> (forall n . a n -> m b n) -> m b n

one :: Const Int n
one = (Const 1)

example_4 :: E (Const Int) n
example_4 = do
  x <- (return one)
  return x

main = example_4 `seq` ()