{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# OPTIONS -Wall #-}
module M where

import Data.Kind
import GHC.TypeLits

data L = L1 | L2

data Foo (xs :: [L]) where
  T1 :: (In L1 xs) => Foo xs
  T2 :: (In L2 xs) => Foo xs

type family In x xs :: Constraint where
  In x (x:xs) = ()
  In x (y:xs) = In x xs
  In x '[] = TypeError (ShowType x :<>: Text "not found in list")

bar :: Foo '[L1] -> ()
bar T1 = ()
bar T2 = ()