:kind! doesn't take UnsaturatedTypeFamilies into account

GHC compiles the following code without issue:

module Bug where

import Data.Proxy

type Id a = a
type X = Proxy Id

However, :kind! chokes when you feed it X:

λ> :kind! X

<interactive>:1:1: error:
    • The type synonym ‘Id’ should have 1 argument, but has been given none
    • In the expansion of type synonym ‘X’