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

import Data.Kind

data MsgDef = MsgDoThis

type family Msg c
  -- where Msg X = MsgDef         -- as a closed tyfam, it always works
type instance Msg X = MsgDef  -- as an open tyfam, it depends on the location of 'data X'

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

type family FromPat c :: Msg c

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

type instance FromPat X = MsgDoThis