[GHC] #14063: Compiling with --backpack with undefined dependency results in "the 'impossible' happened"

[GHC]

#14063: Compiling with --backpack with undefined dependency results in "the
'impossible' happened"
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        Reporter:  rcook             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  Backpack
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by rcook):

 * Attachment "foo.bkp" added.

 Example Backpack file that demonstrates bug

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14063>
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