Type family not reduced, again

This one looks similar to #12381 (closed), but unfortunately, it does not seem to be fixed in HEAD:

{-# LANGUAGE TypeInType, TypeFamilies, FlexibleInstances #-}

import GHC.Types

type family F (a :: Type) :: Type

class C a where
  type D (a :: Type) :: F a

instance (F a ~ Bool) => C a where
  type D a = True

This yields (in 8.0.1 and 8.1.20160709):

Constraint.hs:11:14: error:
    • Expected kind ‘F a’, but ‘'True’ has kind ‘Bool’
    • In the type ‘True’
      In the type instance declaration for ‘D’
      In the instance declaration for ‘C a’