--warn-unreachable sometimes flags sys.version_info checks as unreachable in class methods

[ammaraskar]

When using a sys.version_info check inside a method or a function, the --warn-unreachable flag will sometimes mark version checks that fail the current python version as unreachable.

To Reproduce

import sys


class Foo:
    def less_than_check(self) -> str:
        if sys.version_info < (3, 11):
            return "3.10 or below"
        return "3.11+"
        
    def greater_than_equal(self) -> str:
        if sys.version_info >= (3, 11):
            return "3.11+"
        return "3.10 or below"
        

if sys.version_info < (3, 11):
    print("3.10 or below")
else:
    print("3.11+")
    
if sys.version_info >= (3, 11):
    print("3.11+")
else:
    print("3.10 or below")

See this example of running this snippet on the playground with python version 3.11 and then python version 3.10

Expected Behavior

mypy marks the branches with sys.version_info inside the less_than_check and greater_than_equal that don't match the current version as unreachable. However, it does not mark the free-standing if sys.version_info < (3, 11): and if sys.version_info >= (3, 11): checks in the file as unreachable.

I would expect that the class method and function checks should not get marked unreachable either.

Your Environment

• Mypy version used: 1.14.1
• Mypy command-line flags: --warn-unreachable

[ammaraskar]

Might be related to #12409