[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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Comment (by Ben Gamari <ben@…>):

 In [changeset:"c3aea39678398fdf88166f30f0d01225a1874a32/ghc" c3aea39/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="c3aea39678398fdf88166f30f0d01225a1874a32"
 Fix #14934 by including axSub0R in typeNatCoAxiomRules

 For some reason, `axSub0R` was left out of `typeNatCoAxiomRules` in
 `TcTypeNats`, which led to disaster when trying to look up `Sub0R` from
 an interface file, as demonstrated in #14934.

 The fix is simple—just add `axSub0R` to that list. To help prevent
 an issue like this happening in the future, I added a
 `Note [Adding built-in type families]` to `TcTypeNats`, which
 contains a walkthrough of all the definitions in `TcTypeNats` you
 need to update when adding a new built-in type family.

 Test Plan: make test TEST=T14934

 Reviewers: bgamari, simonpj

 Reviewed By: simonpj

 Subscribers: simonpj, rwbarton, thomie, carter

 GHC Trac Issues: #14934

 Differential Revision: https://phabricator.haskell.org/D4508
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14934#comment:6>
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