This does not compile:

data Foo s (a :: TYPE k) where
  FooC :: Foo s Int#

However, this does:

data Foo (s :: Type) (a :: TYPE k) where
  FooC :: Foo s Int#

This also works:

data Foo (s :: j) (a :: TYPE k) where
  FooC :: Foo s Int#

These are all of the language pragmas I have enabled:

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE UnboxedSums #-}
{-# LANGUAGE UnboxedTuples #-}
{-# LANGUAGE MagicHash #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}

This is pretty easy to work around, but I think it's an incorrect behavior.