{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE DataKinds #-}

class C1 (a :: Bool {- 1 -}) where
    c :: proxy a -> Int

class C2 (a :: [ * ])

class C3 (a :: [(Int, Double)])

class C4 (a :: ( * ))

data X (a :: [*])

x1 = undefined :: X '[Int]
x2 = undefined :: X '[Int, Double]

data Y (a :: (*, Bool))

y1 = undefined :: Y '(Double, True)
y2 = undefined :: Y '(Double, 'False {-comment-})