[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 Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"560bc289fc6a5b308f985d4c84e0cdf1f88c55fd/ghc"
 560bc289/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="560bc289fc6a5b308f985d4c84e0cdf1f88c55fd"
 Revert "Remove unnecessary isTyVar tests in TcType"

 Summary:
 This reverts commit a0899b2f66a4102a7cf21569889381446ce63833. This is
 because
 removing these checks prompts panics in at least two different programs
 reported in #12785.

 Test Plan: ./validate

 Reviewers: simonpj, goldfire, bgamari, austin

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D2931

 GHC Trac Issues: #12785
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12785#comment:13>
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