[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:  infoneeded
        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):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 That being said, I have a wild hunch what could be causing this.
 [http://git.haskell.org/ghc.git/blob/0693b0b0500236a3dec933350a13f1b0e8c1cb54:/compiler/iface/TcIface.hs#l1336
 Here] is where `go_axiom_co` is defined:

 {{{#!hs
 tcIfaceCo :: IfaceCoercion -> IfL Coercion
 tcIfaceCo = go
   where
     -- ...
     go (IfaceAxiomRuleCo ax cos) = AxiomRuleCo <$> go_axiom_rule ax
                                                <*> mapM go cos
     -- ...

     go_axiom_rule :: FastString -> IfL CoAxiomRule
     go_axiom_rule n =
       case Map.lookup n typeNatCoAxiomRules of
         Just ax -> return ax
         _  -> pprPanic "go_axiom_rule" (ppr n)
 }}}

 It seems that the `Sub0R` axiom isn't in
 [http://git.haskell.org/ghc.git/blob/0693b0b0500236a3dec933350a13f1b0e8c1cb54:/compiler/typecheck/TcTypeNats.hs#l378
 typeNatCoAxiomRules]:

 {{{#!hs
 typeNatCoAxiomRules :: Map.Map FastString CoAxiomRule
 typeNatCoAxiomRules = Map.fromList $ map (\x -> (coaxrName x, x))
   [ axAddDef
   , axMulDef
   , axExpDef
   , axLeqDef
   , axCmpNatDef
   , axCmpSymbolDef
   , axAppendSymbolDef
   , axAdd0L
   , axAdd0R
   , axMul0L
   , axMul0R
   , axMul1L
   , axMul1R
   , axExp1L
   , axExp0R
   , axExp1R
   , axLeqRefl
   , axCmpNatRefl
   , axCmpSymbolRefl
   , axLeq0L
   , axSubDef
     -- axSub0R isn't here!!!
   , axAppendSymbol0R
   , axAppendSymbol0L
   , axDivDef
   , axDiv1
   , axModDef
   , axMod1
   , axLogDef
   ]
 }}}

 So perhaps we should just add it.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14934#comment:2>
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