The following program gives a hopelessly confusing error message on GHC 8.6.3 and HEAD:

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

import Data.Kind

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

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

Bug.hs:11:18: error:
    • Couldn't match kind ‘k1’ with ‘k1’
    • In the second argument of ‘SameKind’, namely ‘b’
      In the class declaration for ‘C’
   |
11 | class SameKind a b => C (k :: Const Type a) (b :: k)
   |                  ^

I imagine that the real issue is that SameKind a b would force a :: k, which would be ill-scoped. But figuring that out from this strange error message requires a lot of thought.