@classmethod
def as_decorator(cls, func=None, **kwargs):
    """
        A decorator for validating the units of arguments to functions.

        Unit specifications can be provided as keyword arguments to the decorator,
        or by using function annotation syntax. Arguments to the decorator
        take precedence over any function annotations present.

        A `~astropy.units.UnitsError` will be raised if the unit attribute of
        the argument is not equivalent to the unit specified to the decorator
        or in the annotation.
        If the argument has no unit attribute, i.e. it is not a Quantity object, a
        `ValueError` will be raised unless the argument is an annotation. This is to
        allow non Quantity annotations to pass through.

        Where an equivalency is specified in the decorator, the function will be
        executed with that equivalency in force.

        Notes
        -----

        The checking of arguments inside variable arguments to a function is not
        supported (i.e. \\*arg or \\**kwargs).

        Examples
        --------

        .. code-block:: python

            import astropy.units as u
            @u.quantity_input(myangle=u.arcsec)
            def myfunction(myangle):
                return myangle**2


        .. code-block:: python

            import astropy.units as u
            @u.quantity_input
            def myfunction(myangle: u.arcsec):
                return myangle**2

        Also you can specify a return value annotation, which will
        cause the function to always return a `~astropy.units.Quantity` in that
        unit.

        .. code-block:: python

            import astropy.units as u
            @u.quantity_input
            def myfunction(myangle: u.arcsec) -> u.deg**2:
                return myangle**2

        Using equivalencies::

            import astropy.units as u
            @u.quantity_input(myenergy=u.eV, equivalencies=u.mass_energy())
            def myfunction(myenergy):
                return myenergy**2

        """
    self = cls(**kwargs)
    if func is not None and not kwargs:
        return self(func)
    else:
        return self
