[GHC] #12156: -fdefer-typed-holes causes panic on unbound variable

[GHC]

#12156: -fdefer-typed-holes causes panic on unbound variable
-------------------------------------+-------------------------------------
           Reporter:  osa1           |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         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:
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 {{{#!haskell
 module Main where

 main = do
     print v
     return ()
 }}}

 Compile it without {{{-fdefer-typed-holes}}} and it fails as expected:

 {{{
 ➜  hole_bug ghc --make Main.hs -fforce-recomp
 [1 of 1] Compiling Main             ( Main.hs, Main.o )

 Main.hs:4:11: error: Variable not in scope: v
 }}}

 Adding {{{-fdefer-typed-holes}}} is causing a panic instead of runtime
 error:

 {{{
 ➜  hole_bug ghc --make Main.hs -fforce-recomp -fdefer-typed-holes
 [1 of 1] Compiling Main             ( Main.hs, Main.o )

 Main.hs:4:11: warning: [-Wtyped-holes] Variable not in scope: v
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for x86_64-unknown-linux):
         StgCmmEnv: variable not found
   $dShow_aKo
   local binds for:
   $trModule
   $trModule1_r16k
   $trModule2_r16s
   sat_s16x
   sat_s16z

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

 Tried with: 8.0.1, HEAD as of today.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12156>
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