mypy plugin: teach mypy that validator methods are classmethods

[DMRobertson]

The docs explain that validator functions take a type object as their first argument---cls rather than self.

validators are "class methods", so the first argument value they receive is the UserModel class, not an instance of UserModel.

(This seems to be a small lie---Pydantic internally calls classmethod on the decorated function---but let's ignore that.)

With this in mind, I tried to annotate a validator as follows:

from pydantic import BaseModel, validator
from typing import Type

class Model(BaseModel):
    x: str
    @validator("x")
    def validate_x(cls: Type["Model"], value: str) -> str:
        return value

Model(x = "abc")

This works great---but mypy isn't happy, even using Pydantic's mypy plugin.

$ cat mypy.ini
[mypy]
plugins = pydantic.mypy

$ mypy temp.py
temp.py:7: error: The erased type of self "Type[temp.Model]" is not a supertype of its class "temp.Model"

After removing the annotation for cls, reveal_type(cls) prints temp.Model; I think mypy is analysing the pre-decoration validate_x as if it were a regular method rather than a classmethod. If we instead write

    @validator("x")
    @classmethod
    def validate_x(cls: Type["Model"], value: str) -> str:

Then mypy is happy.

It'd be great if the mypy plugin could do something similar under the covers.

[DMRobertson]

This was done in #4102.