[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
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        Reporter:  deech             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1-rc2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by deech:

@@ -26,2 +26,1 @@
- The bug is a GUI binding. Reproducing is reliable but unfortunately takes
- some setup. To reproduce:
+ The bug is in a demo for a GUI binding. To reproduce:

New description:

 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 in a demo for a GUI binding. 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
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13708#comment:1>
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