[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 simonpj):

 I'm not up to reproducing using docker etc (but perhaps someone else may
 be?).  But here are some pointers.

 * After this:
 {{{
 [599 of 654] Compiling GI.Gtk.Objects.Widget ( GI/Gtk/Objects/Widget.hs,
 .stack-work/dist/x86_64-linux/Cabal-1.24.0.0/build/GI/Gtk/Objects/Widget.o
 )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for x86_64-unknown-linux):
         urk! lookup local fingerprint
   $fAttrInfoWidgetMarginTopPropertyInfo4
 }}}
   if you compile just that one module `GI.Gtk.Objects.Widget` all by
 itself (use `-v` to see the command line that stack is using, I guess),
 does the error repeat?  I hope so!

 * Add `-ddump-simpl` to the command line, which should show the Core that
 `MkIface` stumbles on.

 Beyond that I think we may need to start add debug-prints to `MkIface` to
 get more data. I'm guessing that the strongly-connected component analysis
 in `MkIface.addFingerprints` is somehow missing an edge.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12168#comment:7>
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