[GHC] #12168: panic! (the 'impossible' happened) with gi-gtk 3.0.4

[GHC]

#12168: panic! (the 'impossible' happened) with gi-gtk 3.0.4
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        Reporter:  drb226            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      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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Comment (by inaki):

 (Maintainer of `haskell-gi` here.)

 The bindings are indeed autogenerated. To generate a package that includes
 the code giving rise to the panic, run (in the machine/docker image where
 you get the panic)
 {{{
 $ wget https://hackage.haskell.org/package/gi-gtk-3.0.4/gi-
 gtk-3.0.4.tar.gz
 $ tar xvzf gi-gtk-3.0.4.tar.gz
 $ cd gi-gtk-3.0.4
 $ cabal configure
 $ cabal clean
 $ cd ..
 $ tar -czf gi-gtk-3.0.4.tar.gz gi-gtk-3.0.4
 }}}
 This will generate a tarball of the package including the autogenerated
 code, which you can upload somewhere for examination. Note that further
 `cabal configure`s will not rewrite the autogenerated code (located in the
 `GI` directory).

 (Also, for what it is worth: I cannot reproduce the panic on my computer
 either.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12168#comment:5>
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