[GHC] #15725: Core Lint error: Trans coercion mis-match

[GHC]

#15725: Core Lint error: Trans coercion mis-match
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15703            |  Differential Rev(s):  Phab:D5217
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Replying to [comment:5 simonpj]:
 > Is the bug in [https://www.microsoft.com/en-us/research/publication
 /evidence-normalization-system-fc-2/ the paper], or just in the
 implementation?

 I'm not sure. Having skimmed the paper, it looks like the code I changed
 roughly corresponds to the RedInstTy reduction rule from Figure 7 (when
 you have `InstCo (ForAllCo (a:eta). gamma) phi`). But the RedInstTy rule
 that is shown in the paper is much simpler than the thing that's actually
 implemented in GHC, since the paper doesn't distinguish between the cases
 where `a` is a type variable or coercion variable, nor does it include any
 details of the "lifting context" business from //System FC with Explicit
 Kind Equality//. So I can't really make any kind of judgment on whether
 //Evidence Normalisation in System FC// is correct or not.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15725#comment:6>
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