warn-unreachable behaves differently on else-return than on bare return

Bug Report

mypy --warn-unreachable will error on an unreachable return after an if block but not a semantically equivalent else: return ... after an if block. This shows up in platform checking logic, as an example of "unreachable code" which mypy understands and allows.

Based on what I found in existing warn-unreachable issues, there's no report of this same issue.

To Reproduce

Run mypy --warn-unreachable on the two following samples.

This passes:

def foo():
    if sys.platform != "win32":
        return 1
    else:
        return 0

This fails (on Linux/macOS. Use == "win32" to observe on Windows):

def foo():
    if sys.platform != "win32":
        return 1
    return 0

I also put together a small example dir of this in a github repo, if that makes it easier:
https://github.com/sirosen/repro/tree/2a56d3b/mypy-issues

Expected Behavior

mypy should pass on both of these, as they are the same.

Your Environment

• Mypy version used: 0.910

Inizia eseguendo i due esempi con mypy --warn-unreachable e confronta il modo in cui il type checker gestisce il return vuoto e il ramo else. Traccia l’analisi del codice irraggiungibile e aggiungi una copertura di regressione per i controlli della piattaforma; il lavoro è completato quando entrambi gli esempi semanticamente equivalenti ricevono il risultato previsto.