Unprincipled defaults

[David Feuer]

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?
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