[GHC] #13242: Panic "StgCmmEnv: variable not found" with ApplicativeDo and ExistentialQuantification

[GHC]

#13242: Panic "StgCmmEnv: variable not found" with ApplicativeDo and
ExistentialQuantification
-------------------------------------+-------------------------------------
           Reporter:  ljli           |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{#!hs
 -- 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
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13242>
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