[GHC] #15370: Typed hole panic on GHC 8.6 (tcTyVarDetails)

[GHC]

#15370: Typed hole panic on GHC 8.6 (tcTyVarDetails)
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        Reporter:  RyanGlScott       |                Owner:  Tritlo
            Type:  bug               |               Status:  merge
        Priority:  high              |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.4.3
  checker)                           |             Keywords:  TypeInType,
      Resolution:                    |  TypedHoles
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 Hmm, I'm afraid there is a piece that we are missing on `ghc-8.6`.
 `T15370` fails with
 {{{
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.6.0.20180802 for x86_64-unknown-linux):
         tcTyVarDetails
   co_a1vS :: y_a1vK[sk:1] ~# x_a1vJ[sk:1]
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1164:37 in
 ghc:Outputable
         pprPanic, called at compiler/basicTypes/Var.hs:497:22 in ghc:Var

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}
 when this is cherry-picked on `ghc-8.6`.

 Tritlo, do you think you could look into this?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15370#comment:15>
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