GHC hangs on this (it was panicking on the same code before : https://github.com/goldfirere/ghc/issues/56(https://github.com/goldfirere/ghc/issues/56) :

{-# LANGUAGE GADTs, TypeInType #-}
import Data.Kind
data A :: Type where
  B :: forall (a :: A). A

I guess the typechecker tries to infer the kind of A from the type of the constructors and hangs. Maybe recursive occurrences of a type as a kind in its constructors should only be allowed when its kind signature is specified and can be used to typecheck the constructors.