False negative for non-overlapping type check inside function with strict_equality flag #10055

Bug Report

To Reproduce

Run this file to test.py:

if 'red' == 5:  # <-- Mypy finds error
    print('equal')

def foo():
    if 'red' == 5:  # <-- Mypy does not find error
        print('equal')

foo()

then execute

mypy test.py --strict-equality

Expected Behavior

Mypy should find 2 errors in this file.

Actual Behavior

Mypy fails to find the error inside the function.

Your Environment

• Mypy version used: 0.8000
• Mypy command-line flags: --strict-equality
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.9.1
• Operating system and version: Ubuntu 18.04