Check for fixed runtime rep fails unexpectedly when runtime rep is determined by default signature constraint.

I wondered if we could allow Unlifted instances for things like Eq and ran into this issues:

class Lifted a where
instance Lifted (a :: Type)

class Eq (a :: TYPE (BoxedRep r)) where
    (==) :: a -> a -> Bool
    (/=) ::  a -> a -> Bool
    default (/=) :: Lifted a => a -> a -> Bool
    (/=) x y = not $ x == y

Lifted constrains the default method to lifted types. And hence the runtime rep for the default method should be fixed. But I get this error:

Test.hs:24:5: error: [GHC-55287]
    The second pattern in the equation for ‘/=’
    does not have a fixed runtime representation.
    Its type is:
      a :: TYPE (BoxedRep r)
   |
24 |     (/=) x y = not $ x == y
   |     ^^^^^^^^^^^^^^^^^^^^^^^

I can't say I'm surprised that it doesn't work. But it would also be nice if it would work.