`sys.platform != "foo"` defeats platform-specific path

[Jacobfaib]

import sys


def foo() -> None:
    if sys.platform != "darwin":
        print("not darwin")
        return

    print("darwin")

$ mypy --platform linux plat.py
plat.py: note: In function "foo":
plat.py:9:5: error: Statement is unreachable  [unreachable]
        print("darwin")
        ^~~~~~~~~~~~~~~
Found 1 error in 1 file (checked 1 source file)

mypy claims to understand sys.platform checks, yet is defeated by !=.

I would expect mypy not to warn about unreachable code here:

1. I am not assigning sys.platform to an intermediate
2. I am checking against a literal value (which in the documentation mypy also claims to understand).

[brianschubert]

Thanks for the report. Platform checks currently only apply within the if/elif/else statement. If you use an explicit else branch, this will work as expected:

import sys


def foo() -> None:
    if sys.platform != "darwin":
        print("not darwin")
        return
    else:
        print("darwin")  # OK

Closing as duplicate of #10773