[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:
       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:                    |
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Comment (by tdammers):

 So far, a naive attempt at rewriting `tcvs_of_type` in terms of `VarSet ->
 VarSet` didn't produce significant performance improvements; however, I
 have not eliminated `unionVarSet` completely yet - the `ForAll` cases in
 both `tcvs_of_type` and `tcvs_of_co` are both such that naively expressing
 them as compositions of applications of `tcvs_of_...` and `delVarSet` does
 not produce correct results - I assume this is because the order in which
 vars are added and deleted from the set matter. So I'm now testing a
 replacement for `delVarSet` that is written in terms of set insertions
 instead of set unions; hopefully this will show a bigger difference. I
 should probably also benchmark the relevant set operations on their own,
 to verify that they do indeed expose the performance issue we're
 suspecting.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14880#comment:49>
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