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

[GHC]

#14934: Repeated "impossible" go_axiom_rule error.
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           Reporter:  galen          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.4.1
           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):                 |         Wiki Page:
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 I am getting the following error repeatedly:

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.4.1 for x86_64-unknown-linux):
         go_axiom_rule
   Sub0R
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in
 ghc:Outputable
         pprPanic, called at compiler/iface/TcIface.hs:1371:15 in
 ghc:TcIface
 }}}

 I got it on 8.2.1, and tried upgrading to 8.4.1 to see if it went away,
 but it did not.

 It occurs frequently when I do a `--make` and recompile a subset of
 modules.  I can avoid it by force-recompiling all modules, although of
 course this is inefficient.

 Since I'm working with a codebase of thousands of lines and don't
 understand ghc's innards, I'm not sure where to start looking for the
 cause, to produce a minimal failing example.  But it did start happening
 around the time I expanded my use of this module:

 https://github.com/agrafix/superrecord

 Since there are unsafe operations in there, it is possible it is doing
 something illicit, but it is hard to see how it would cause this.

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