Bug Report

This is about the type narrowing logic updated in 1.20.0: == and in may cause a variable to lose its type: when checked against a type var, these checks cause the lhs variable to assume the type of the rhs. In general the resulting type should probably be some kind of intersection. Without such a construct there are certainly situations where the current behavior might be expected (and it's certainly correct where the variable is of a type that's a supertype of the one checked against), but also where this will be quite confusing.

To Reproduce

from typing import Hashable, TypeVar

H = TypeVar('H', bound=Hashable)

def func(y: H):
    x: int

    assert x == y
    reveal_type(x)  # Revealed type is "H"
    x += 1  # Unsupported operand types for + ("H" and "int")

Playground

So, the check causes x to lose its type.

Expected Behavior

No error to be thrown.

Your Environment

• Mypy version used: 1.20.0
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.11.12