Constraint synonym instances

Something funny happens when you try to declare an instance of a constraint synonym:

{-# LANGUAGE ConstraintKinds #-}
module F where

type ShowF a = Show (a -> Bool)

instance ShowF Int where
    show _ = "Fun"

I get:

F.hs:8:5: error: ‘show’ is not a (visible) method of class ‘ShowF’
  |
8 |     show _ = "Fun"
  |     ^^^^

OK, but it gets weirder. Look at:

{-# LANGUAGE ConstraintKinds, FlexibleContexts, FlexibleInstances #-}
module F where

type ShowF a = (Show (a -> Bool))

instance ShowF Int where

This is accepted (with a complaint that show is not implemented.) It gets even more awful:

{-# LANGUAGE ConstraintKinds, FlexibleContexts, FlexibleInstances, MultiParamTypeClasses #-}
module F where

type ShowF a = (Show Bool, Show Int)

instance ShowF Int where

This is awful: GHC treats Show Bool and Show Int as if they were constraints, and then emits the following DFun:

df9d1b4635f2a752f29ff327ab66d1cb
  $f(%,%)ShowShow :: (Show Bool, Show Int)
  DFunId
  {- Strictness: m, Inline: CONLIKE,
     Unfolding: DFun: @ a @ b.
                  @ (Show Bool) @ (Show Int) $fShowBool $fShowInt -}

I don't even know what this is supposed to mean.

OK, so what should we do? I think there are a few possibilities:

1. Completely outlaw instance declarations on constraint synonyms.
2. Allow instance declarations on constraint synonyms, but only if after desugaring the synonym, you end up with a single class head. I would find this useful in a few cases, for example, if you are writing instance MonadSample (Impl t) MyMonad, if you had type MonadSample2 t a = MonadSample (Impl t) a you might prefer writing instance MonadSample2 t MyMonad instead
3. Figure out what to do with instance declarations with multiple class heads, and proceed accordingly.