[GHC] #15552: Infinite loop/panic with an existential type.

[GHC]

#15552: Infinite loop/panic with an existential type.
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        Reporter:  howtonotwin       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:  TypeInType,
                                     |  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14723            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Another idea for (1): just eliminate the "optimisation" in
 `TcHsType.zonkTyVarOcc`.   Types generally have no sharing, so we may not
 be gaining much by increasing sharing her.

 NB: in `TcMType.zonkTcTyVar` we could apply this optimisation; for some
 reason we don't even try to do that too.

 Another idea: maintain (statefully) a map from (meta) `TyVar` to `TyVar`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15552#comment:8>
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