Unprincipled defaults

On occasion, it can be useful to have default definitions that don't typecheck even with DefaultSignatures. It would be nice to be able to use them anyway. For example, if we have class Semigroup1 f where op :: f a -> f a -> f a then we could, hypothetically, give a default definition for (<>): default (<>) :: Semigroup1 f => f a -> f a -> f a (<>) = op But we can't give a default definition op = (<>) because there's no way to write its signature. However, for any F with instance Semigroup (F a) where ... the definition op = (<>) is perfectly fine. Would it be possible to offer a completely wild defaulting mechanism allowing *any expression* as a default, and only checking its type and compiling it when it's actually used?