Panic "StgCmmEnv: variable not found" with ApplicativeDo and ExistentialQuantification

-- Panic.hs
{-# LANGUAGE ApplicativeDo #-}
{-# LANGUAGE ExistentialQuantification #-}
module Panic where

import Data.STRef
import Control.Monad.ST

data A = forall a. A a

st :: ST s ()
st = do
      A _ <- pure $ A True
      ref <- newSTRef 1
      readSTRef ref
      pure ()

$ ghc Panic.hs
[1 of 1] Compiling Panic            ( Panic.hs, Panic.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.1.20170106 for x86_64-unknown-linux):
	StgCmmEnv: variable not found
  $dMonad_aGQ
  local binds for:
  $trModule
  $tcA
  $tc'A
  $tcA1_rSi
  $tc'A1_rSJ
  $trModule1_rSK
  $trModule2_rSL
  sat_sT5
  sat_sT9
  sat_sTa
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
        callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable
        pprPanic, called at compiler/codeGen/StgCmmEnv.hs:137:9 in ghc:StgCmmEnv

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

The above module fails to compile, giving the pasted panic message. Without the ApplicativeDo pragma compilation succeeds. Reproduced with GHC 8.0.2 and a recent head, courtesy of nixpkgs:

gp84vpgar3n3rqvkwj47yyhxvsvbsmi6-ghc-8.1.20170106

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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