[GHC] #12785: GHC panic, `tcTyVarDetails` is missing a case

[GHC]

#12785: GHC panic, `tcTyVarDetails` is missing a case
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        Reporter:  heisenbug         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:  yes
      Blocked By:                    |             Blocking:
 Related Tickets:  #12590            |  Differential Rev(s):  Phab:D2931
       Wiki Page:                    |
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Comment (by goldfire):

 For the record, I'm fully agreed with comment:11.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12785#comment:12>
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