[GHC] #14880: GHC panic: updateRole

[GHC]

#14880: GHC panic: updateRole
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        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15076            |  Differential Rev(s):  Phab:D4769,
       Wiki Page:                    |  Phab:D5141
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Comment (by goldfire):

 * First off, no mists of time here: the more ambitious patch was because
 comment:13 fixed the problem in TcType -- which caused the original bug --
 but that same problem existed in TyCoRep. We just didn't have (and still
 don't, to my knowledge) a concrete program that exhibits misbehavior. I
 don't remember the exact cause of the bug I mentioned in comment:13, but I
 don't think that's about the changes in TyCoRep.

 * Your new approach relies on this invariant: If a variable in a tyvar
 kind is in the bound-var set, then the variable is in the bound-var set,
 too. Perhaps easier is to think about the contrapositive: If a variable is
 not in the bound-var set (in other words, if we look at it at all), then
 no variable in its kind is in the bound-var set (so we can zap the bound-
 var set). This invariant is certainly true of well-typed programs. (It's
 just barely conceivable that it might not be true in the case of a mis-
 ordered telescope, but let's not worry about that now.) So: yes, I think
 that works nicely.

     Nice idea. It's very much worth a Note.

 @tdammers: Do you think you can push this new idea through? I can write
 the Note if need be.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14880#comment:124>
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