[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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Changes (by RyanGlScott):

 * differential:   => Phab:D2931


Comment:

 I tried poking around the code from
 a0899b2f66a4102a7cf21569889381446ce63833. Interestingly, adding back the
 explicit `isTyVar` check to `isFloatedTouchableMetaTyVar` fixes the
 program in comment:7, but //not// heisenbug's program.

 I could try to hunt for the other function that trips up heisenbug's
 program, but chances are there other undiscovered programs that would trip
 up this panic through other means. Therefore, to minimize the chances of
 that happening, I've opted to open Phab:D2931, which simply reverts
 a0899b2f66a4102a7cf21569889381446ce63833.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12785#comment:9>
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