: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’