GHC panic (No skolem info) with RankNTypes and strange scoping

The following program panics with GHC 8.6.3 and HEAD:

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE QuantifiedConstraints #-}
module Bug where

import Data.Kind

type Const a b = a
type SameKind (a :: k) (b :: k) = (() :: Constraint)
class (forall (b :: k). SameKind a b) => C (k :: Const Type a)

$ /opt/ghc/8.6.3/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:11:36: error:ghc: panic! (the 'impossible' happened)
  (GHC version 8.6.3 for x86_64-unknown-linux):
        No skolem info:
  [k1_a1X4[sk:1]]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable
        pprPanic, called at compiler/typecheck/TcErrors.hs:2891:5 in ghc:TcErrors

As with #16244 (closed), I imagine that the real culprit is that SameKind a b would force a :: k, which would be ill-scoped.

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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