def _structure_default(self, obj, cl):
        """This is the fallthrough case. Everything is a subclass of `Any`.

        A special condition here handles ``attrs`` classes.

        Bare optionals end here too (optionals with arguments are unions.) We
        treat bare optionals as Any.
        """
        if cl is Any or cl is Optional:
            return obj
        # We don't know what this is, so we complain loudly.
        msg = (
            "Unsupported type: {0}. Register a structure hook for "
            "it.".format(cl)
        )
        raise ValueError(msg)