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

[GHC]

#15778: GHC HEAD-only panic (zonkTcTyVarToTyVar)
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.8.1
          Component:  Compiler       |           Version:  8.5
  (Type checker)                     |
           Keywords:                 |  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:
-------------------------------------+-------------------------------------
 The following program typechecks on GHC 8.0.1 through 8.6.1:

 {{{#!hs
 {-# LANGUAGE AllowAmbiguousTypes #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeOperators #-}
 module Bug where

 import Data.Kind

 type a ~> b = a -> b -> Type
 type family Apply (f :: k1 ~> k2) (x :: k1) :: k2
 data family Sing (a :: k)

 data Flarble (a :: Type) where
   MkFlarble :: Flarble Bool
 data instance Sing (z :: Flarble a) where
   SMkFlarble :: Sing MkFlarble

 elimFlarble :: forall a
                       (p :: forall x. Flarble x ~> Type)
                       (f :: Flarble a).
                Sing f
             -> Apply p MkFlarble
             -> Apply p f
 elimFlarble s at SMkFlarble pMkFlarble =
   case s of
     (_ :: Sing (MkFlarble :: Flarble probablyABadIdea)) ->
       pMkFlarble
 }}}

 However, it panics on GHC HEAD:

 {{{
 $ ~/Software/ghc/inplace/bin/ghc-stage2 Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.7.20181015 for x86_64-unknown-linux):
         zonkTcTyVarToTyVar
   probablyABadIdea_aWn[tau:2]
   Bool
   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
 }}}

 If I replace `probablyABadIdea` with `Bool`, then it typechecks again.

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