[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 drb226):

 Note that re-running the {{{stack build}}} command in either of these
 scenarios does seem to move the build along without resurfacing the panic.
 However, a fresh {{{stack unpack}}} will consistently lead to producing
 the panic the first time you try to build it.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12168#comment:1>
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