Can't resolve superclass from MPTC

Summary

If I have a Functor constraint, I'm told it's redundant. If I remove it, I get a (rather poor) error message. This is reduced from production code.

Steps to reproduce

{-# language MultiParamTypeClasses #-}
{-# language UndecidableInstances #-}
{-# options_ghc -Wredundant-constraints #-}
module Super where

newtype KeyAttributes = KeyAttributes { keySort :: Int }
class Functor base => Synthetic syn base where
  synthetic :: base syn -> syn
instance (Synthetic Int base, {- Functor base -}) => Synthetic KeyAttributes base where
  synthetic base = KeyAttributes { keySort = synthetic (keySort <$> base) }

With Functor base commented out, I get

Super.hs:7:10: error:
    • Could not deduce (Functor base)
        arising from the superclasses of an instance declaration
      from the context: Synthetic Int base
        bound by the instance declaration at Super.hs:7:10-59
      Possible fix:
        add (Functor base) to the context of the instance declaration
    • In the instance declaration for ‘Synthetic KeyAttributes base’
  |
7 | instance Synthetic Int base => Synthetic KeyAttributes base where

If I uncomment that, I get

Super.hs:7:10: warning: [-Wredundant-constraints]
    • Redundant constraint: Functor base
    • In the instance declaration for ‘Synthetic KeyAttributes base’
  |
7 | instance (Synthetic Int base, Functor base) => Synthetic KeyAttributes base where
  |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Expected behavior

If reasonably implementable, I'd like the code to compile with no warnings without a Functor constraint. Otherwise, I don't want a warning with it, and I want a better explanation for why it can't be inferred without.

Environment

• GHC version used: 9.4.4

Optional:

• Operating System:
• System Architecture: