[GHC] #13959: substTyVar's definition is highly dubious

[GHC]

#13959: substTyVar's definition is highly dubious
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 Thanks for digging into this further. My question is: where do the
 arguments to `mkVarSet` come from? Why don't they already have the kind
 variables in the lists? (They probably should!) My hunch is that the
 assertion failures you're seeing are genuine -- that is, they suggest real
 latent bugs.

 So, I propose this solution: keep digging! :)

 More concretely, point us to a small sampling (1 might be a suitable small
 number) of places where this `mkVarSet` call happens, and see if you can
 describe the context for where that list of variables comes from.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13959#comment:1>
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