[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"
-------------------------------------+-------------------------------------
           Reporter:  rcook          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           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:
-------------------------------------+-------------------------------------
 Using GHC 8.2.1:

 {{{
 > ghc --version
 The Glorious Glasgow Haskell Compilation System, version 8.2.1
 }}}

 And compiling with `--backpack` as follows:

 {{{
 > ghc --backpack foo.bkp
 [1 of 4] Processing foo-indef
   [1 of 2] Compiling Str[sig]         ( foo-indef\Str.hsig, nothing )
   [2 of 2] Compiling Foo              ( foo-indef\Foo.hs, nothing )
 [2 of 4] Processing foo-string
   Instantiating foo-string
   [1 of 1] Compiling Str              ( foo-string\Str.hs, foo-
 string\Str.o )
 [3 of 4] Processing foo-int
   Instantiating foo-int
   [1 of 1] Compiling Str              ( foo-int\Str.hs, foo-int\Str.o )
 [4 of 4] Processing main
 ghc.EXE: panic! (the 'impossible' happened)
   (GHC version 8.2.1 for x86_64-unknown-mingw32):
         no package name
 CallStack (from HasCallStack):
   error, called at compiler\backpack\DriverBkp.hs:573:32 in ghc:DriverBkp

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

 make: *** [foo-indef/Foo.hi] Error 1
 }}}

 See attached `foo.bkp` file. While the Backpack file ''is'' invalid, in
 that the `main` unit mentions the `foo` dependency, which does not exist,
 this shouldn't lead to a GHC panic.

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