The well-kinded type invariant (in TcType)

(Originally noticed here.)

The following program typechecks on GHC 8.2.2 on GHC 8.4.1, but panics on GHC HEAD:

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}
module Bug where

import Data.Kind (Type)

data family Sing (a :: k)

newtype instance Sing (f :: k1 ~> k2) =
  SLambda { applySing :: forall t. Sing t -> Sing (Apply f t) }

data TyFun :: Type -> Type -> Type
type a ~> b = TyFun a b -> Type
infixr 0 ~>
type family Apply (f :: k1 ~> k2) (x :: k1) :: k2

class SingI (a :: k) where
  sing :: Sing a

data ColSym1 :: f a -> a ~> Bool
type instance Apply (ColSym1 x) y = Col x y

class PColumn (f :: Type -> Type) where
  type Col (x :: f a) (y :: a) :: Bool

class SColumn (f :: Type -> Type) where
  sCol :: forall (x :: f a) (y :: a).
    Sing x -> Sing y -> Sing (Col x y :: Bool)

instance (SColumn f, SingI x) => SingI (ColSym1 (x :: f a) :: a ~> Bool) where
  sing = SLambda (sCol (sing @_ @x))

$ /opt/ghc/head/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.5.20180201 for x86_64-unknown-linux):
        piResultTy
  k_aZU[tau:1]
  (a_aW8[sk:1] |> <*>_N)
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:947:35 in ghc:Type

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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Operating system
Architecture