{-# LANGUAGE GHC2021 #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
module M where

import Data.Kind
import GHC.TypeLits

data SimplePattern = SP1 | SP2 | SP3 Type

data Msgx = MsgDoThis | MsgDoThat Type

type family Msg c
  where Msg X = Msgx        -- if you uncomment this it compiles
-- type instance Msg X = Msgx  -- if you uncomment this it fails

class D c where
    type FromPat c (sp :: SimplePattern) :: Maybe (Msg c)

data X


instance D X where
    type FromPat X SP1 = 'Just (MsgDoThis :: Msg X)
    type FromPat X SP2 = 'Nothing
    type FromPat X (SP3 t) = 'Just (MsgDoThat t)