False positive with --warn-unreachable when an instance variable is modified inside a for loop #13973

Bug Report

mypy mark as unreachable code that is perfectly reachable, perhaps because it interprets an instance variable typed as Optional to always be None.

To Reproduce

from typing import Optional

# executed with --warn-unreachable

class A:
    x: Optional[int]

    def execute(self) -> None:
        self.x = None
        for i in range(3):
            if self.x is None:
                self.x = 10
            else:
                self.x = 20  # error: Statement is unreachable  [unreachable]
            print(self.x)


a = A()
a.execute()

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Expected Behavior

No errors.

Actual Behavior

Showing: error: Statement is unreachable [unreachable]

Your Environment

• Mypy version used: 0.982
• Mypy command-line flags: --warn-unreachable
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.10