False positive in case of custom __setattr__ that changes the type of the attribute

Bug Report

I have a class with a custom __setattr__(self, attr, value:TYPE). This method converts value to another type.
I am getting false positives, since mypy assumes that for each particular TYPE, type(getattr(obj, attr)) == TYPE.

To Reproduce
Minimal code example:

from typing import *

class Context:
    def __setattr__(self, attr:str, value:Any):
        super().__setattr__(attr, str(value))

ctx = Context()
ctx.a = 10
print(len(ctx.a))

Actual Behavior

mypy output

....py:9: error: "Context" has no attribute "a"
....py:9: error: Argument 1 to "len" has incompatible type "int"; expected "Sized"

I would like to tell mypy that for this class, it mustn't assume that type(getattr(ctx, "a")) == type(10) .

Your Environment

• mypy 0.961 (compiled: yes) on Ubuntu Linux (Jupyter Docker image)
• Python 3.8.10 (Anaconda)

Start by running the minimal Context example through mypy and compare the reported errors with the expected behavior described in the issue. Trace the type-checking path for custom setattr and inferred attributes; done means the example no longer produces false positives for the converted attribute type.