MultiParamTypeClasses + ExtendedDefaultRules

-XExtendedDefaultRules allows us to avoid ambiguity for unannotated numeric literals:

{-# LANGUAGE ExtendedDefaultRules #-}

data A = A deriving Show

class ToA a where
   toA :: a -> A

instance ToA Double where
   toA _ = A

main = print (toA 5 :: A)

But if we have a multi-param typeclass, -XExtendedDefaultRules doesn't help us:

{-# LANGUAGE ExtendedDefaultRules #-}

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}

data A x = A deriving Show

class ToA a x where
   toA :: a -> A x

instance ToA Double x where
   toA _ = A

main = print (toA 5 :: A Bool)

It would be really nice for an EDSL of mine to be able to use extended defaults here! At the moment my only option has been to use incoherent instances, which has resulted in incoherent behavior...