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

Empieza ejecutando los dos ejemplos con mypy --warn-unreachable y compara cómo maneja el comprobador de tipos el return vacío y la rama else. Sigue el análisis del código inalcanzable y añade cobertura de regresión para las comprobaciones de plataforma; estará terminado cuando ambos ejemplos semánticamente equivalentes reciban el resultado esperado.