[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):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * cc: simonpj (added)


Comment:

 That patch did fix building the program in comment:7, FWIW.

 Also, I found out that Simon's commit
 a0899b2f66a4102a7cf21569889381446ce63833 caused the regression:

 {{{
 From a0899b2f66a4102a7cf21569889381446ce63833 Mon Sep 17 00:00:00 2001
 From: Simon Peyton Jones <simonpj at microsoft.com>
 Date: Tue, 1 Mar 2016 17:05:56 +0000
 Subject: [PATCH] Remove unnecessary isTyVar tests in TcType

 These extra tests were added by Richard when he had CoVars floating
 around in places where previously only TyVars had been.  But
 fortunately those days are gone, so these tests are unnecessary,
 and are slowing GHC down.

 Let's remove them.
 }}}

 It looks like Simon has
 [https://ghc.haskell.org/trac/ghc/changeset/f4a14d6c535bdf52b19f441fe185ea13d62fdc24/ghc
 done some digging] on this recently, but I'll cc him just in case.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12785#comment:8>
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