[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):
       Wiki Page:                    |
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Comment (by simonpj):

 >  The trouble is that MkBar has an implicit forall's variable a, whose
 kind mentions an explicit type variable arg.

 Richard and I discussed this on Monday.  Our conclusions

 * `Inferred` variables should always precede `Specified` ones.  That is,
 do not top-sort

 * Idea: simply refrain from quantifying any inferred variables that
 mention specified ones.  They'll get defaulted to `Any` which is probably
 fine.  This refraining can readily be done in `candidatesQTyVarsOfType`.

 * Also came up: close over kinds once in `tyCoVarsOfType` instead of at
 every leaf. This is not just an efficiency issue: consider
 {{{
 tyCoVarsOfType (forall a. b::a)
 }}}
 Richard is on the job

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14880#comment:10>
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