{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE PolyKinds #-}
module M where

import Data.Kind
import GHC.TypeLits

data Msgx = MsgDoThis | MsgDoThat Type

type family Msg c
type instance Msg X = Msgx

data X   -- if you uncomment this definition of X, it compiles

type family FromPat c :: Maybe (Msg c)

-- data X   -- if you uncomment this definition of X, it fails

type instance FromPat X = 'Just (MsgDoThis :: Msg X)