[GHC] #13708: Panic! (the "impossible" happened) bug in GHC 8.2.1 rc2

[GHC]

#13708: Panic! (the "impossible" happened) bug in GHC 8.2.1 rc2
-------------------------------------+-------------------------------------
           Reporter:  deech          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1-rc2
           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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 The bug can be reproduced but it does take some setup. Here's the error
 report in case it's familiar:

 {{{
 ...
 [1 of 2] Compiling Valuators        ( .stack-
 work/dist/x86_64-linux/Cabal-2.0.0.0/build/fltkhs-fluid-valuators/fltkhs-
 fluid-valuators-tmp/Valuators.hs, .stack-
 work/dist/x86_64-linux/Cabal-2.0.0.0/build/fltkhs-fluid-valuators/fltkhs-
 fluid-valuators-tmp/Valuators.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.2.0.20170507 for x86_64-unknown-linux):
         idInfo
   a_ad4D
   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/basicTypes/Var.hs:526:34 in ghc:Var

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

 The bug is a GUI binding. Reproducing is reliable but unfortunately takes
 some setup. To reproduce:

 {{{
 # Install the FLTK GUI toolkit from source.
 > wget http://fltk.org/pub/fltk/1.3.4/fltk-1.3.4-1-source.tar.gz
 > tar -zxf fltk-1.3.4-1-source.tar.gz
 > cd fltk-1.3.4-1
 > ./configure --enable-gl --enable-shared --enable-localjpeg --enable-
 localzlib --enable-localpng
 > make
 > sudo make install
 > fltk-config --version
   1.3.4-1

 # Install the bindings from source
 > git clone -b 8.2.1-rc2 https://github.com/deech/fltkhs
 > cd fltkhs
 > cabal install

 # Install the package causing the bug from source
 > git clone -b 8.2.1-rc2 https://github.com/deech/fltkhs-fluid-demos
 > cd fltkhs-fluid-demos
 > cabal install
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13708>
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