strange error when using data families with levity polymorphism and unboxed sums and data families

I've the following small example

{-# LANGUAGE MagicHash, UnboxedSums, NoImplicitPrelude #-}
{-# LANGUAGE TypeFamilies #-}
-- {-# LANGUAGE PolyKinds #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE GADTs ,ExplicitNamespaces#-}
{-# LANGUAGE UnboxedTuples #-}

module Data.Unboxed.Maybe where

import GHC.Exts
import GHC.Types
import Prelude (undefined)
import Data.Void

data family Maybe(x :: TYPE (r :: RuntimeRep))

data instance Maybe (a :: * ) where
  MaybeSum :: (# a  | (# #) #) -> Maybe a
data instance Maybe (x :: TYPE 'UnliftedRep) where
  MaybeSumU :: (# x | (# #) #) -> Maybe x

and then i get the error (made much saner to read by use of printing explicit kinds)

Prelude> :r
[1 of 1] Compiling Data.Unboxed.Maybe ( src/Data/Unboxed/Maybe.hs, interpreted )

src/Data/Unboxed/Maybe.hs:22:3: error:
    • Data constructor ‘MaybeSumU’ returns type ‘Maybe 'LiftedRep x’
        instead of an instance of its parent type ‘Maybe 'UnliftedRep x’
    • In the definition of data constructor ‘MaybeSumU’
      In the data instance declaration for ‘Maybe’
   |
22 |   MaybeSumU :: (# x | (# #) #) -> Maybe x

this is

a) a case where printing runtime reps makes things easier to debug :)

b) a very confusing type error since the data instance clearly says "x :: TYPE 'UnliftedRep "

is there something i'm overlooking, or is this a bug?

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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