GHC HEAD-only panic (zonkTcTyVarToTyVar)

The following program typechecks on GHC 8.0.1 through 8.6.1:

{-# 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@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.

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler (Type checker)
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