[GHC] #15778: GHC HEAD-only panic (zonkTcTyVarToTyVar)

[GHC]

#15778: GHC HEAD-only panic (zonkTcTyVarToTyVar)
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.8.1
       Component:  Compiler (Type    |              Version:  8.7
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
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 RyanGlScott):

 The following code does //not// compile on earlier versions of GHC, but it
 is the simplest way to trigger the panic that I've discovered:

 {{{#!hs
 {-# LANGUAGE PolyKinds, ScopedTypeVariables #-}
 foo (_ :: (a :: probablyABadIdea)) = ()
 }}}
 {{{
 $ /opt/ghc/head/bin/ghc Bug.hs
 [1 of 1] Compiling Main             ( Bug.hs, Bug.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.7.20181015 for x86_64-unknown-linux):
         zonkTcTyVarToTyVar
   probablyABadIdea_arQ[tau:1]
   TYPE t_arX[tau:1]
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
 ghc:Outputable
         pprPanic, called at compiler/typecheck/TcMType.hs:1627:34 in
 ghc:TcMType
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15778#comment:4>
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