Exactly what it says on the tin. It would be nice if the following would compile:

{-# LANGUAGE DataKinds, KindSignatures, GADTs #-}

import GHC.TypeLits
import GHC.Natural

data X = X String Natural

data Y :: X -> * where
    Y :: Y ('X "hello" 4)

I kind of assume there's already a ticket for this, but couldn't find one, so figured I'd open one in case.