Foo ('A | 'B)

test.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE TypeApplications #-}

data K = A | B | C
data Foo a where
	FooAny :: Foo a
	FooAB :: Foo (Fam a)
	FooC :: Foo C

type family Fam a where
	Fam C = A
	Fam x = x

example = FooAB @A