[GHC] #14934: Repeated "impossible" go_axiom_rule error.

[GHC]

#14934: Repeated "impossible" go_axiom_rule error.
-------------------------------------+-------------------------------------
        Reporter:  galen             |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.4.1
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  Compile-time      |  (amd64)
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4508
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * status:  new => patch
 * differential:   => Phab:D4508


Comment:

 Or, for maximum terseness:

 {{{
 $ ghc Foo.hs -c -O
 $ ghc Bar.hs -c -O
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.2.2 for x86_64-unknown-linux):
         go_axiom_rule
   Sub0R
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
         callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in
 ghc:Outputable
         pprPanic, called at compiler/iface/TcIface.hs:1349:15 in
 ghc:TcIface
 }}}

 I'll use this as the regression test in Phab:D4508.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14934#comment:5>
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