https://docs.python.org/zh-cn/3.8/library/functools.html#functools.singledispatchmethod

as the code:
```
from functools import singledispatchmethod

class Negator:
    @singledispatchmethod
    @classmethod
    def neg(cls, arg):
        raise NotImplementedError("Cannot negate a")

    @neg.register
    @classmethod
    def _(cls, arg: int):
        return -arg

    @neg.register
    @classmethod
    def _(cls, arg: bool):
        return not arg

```

  File "/root/.pyenv/versions/3.8.5/lib/python3.8/functools.py", line 907, in register
    return self.dispatcher.register(cls, func=method)
  File "/root/.pyenv/versions/3.8.5/lib/python3.8/functools.py", line 849, in register
    raise TypeError(
TypeError: Invalid first argument to `register()`: <classmethod object at 0x7fa22c0495b0>. Use either `@register(some_class)` or plain `@register` on an annotated function.

There's an earlier issue created that relates to this (https://bugs.python.org/issue39679) and there are some possible solutions to get singledispatchmethod and classmethod work there.

Thanks,so close this one due to duplicate issue.