[Haskell-cafe] Auto-deriving Control.Exception.Exception

If this is valid: import qualified Control.Exception as Exc genericFromException :: Typeable a => Exc.SomeException -> Maybe a genericFromException (Exc.SomeException e) = cast e instance Exc.Exception SomeType where fromException = genericFromException then why not have an auto-deriving rule for Exception (fromException = cast . unSomeException)?