[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 RyanGlScott):

 Sorry, it turns out I was using `fpco/stack-build`, not
 `snoyberg/stackage:nightly`.

 I don't have the time to dig into this mess right now, but out of
 curiosity, can I be assured that I will have the same build environment on
 the `snoyberg/stackage:nightly` Docker image in the future? The use of the
 tag `nightly` scares me—is there a way to use the digest SHA to ensure
 that I can achieve the same environment later?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12168#comment:4>
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