Truthiness of None Does not Narrow Optional Types

Note: if you are reporting a wrong signature of a function or a class in
the standard library, then the typeshed tracker is better suited
for this report: https://github.com/python/typeshed/issues

Please provide more information to help us understand the issue:

• Are you reporting a bug, or opening a feature request?
  I believe this is a bug

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

from typing import Optional

def the_method(maybe_int: Optional[int]):
    can_divide = bool(maybe_int)
    if can_divide:
        100 / maybe_int

• What is the actual behavior/output?

error: Unsupported operand types for / ("int" and "None")

• What is the behavior/output you expect?
  I would think that the conditional branch would recognize that only int values would ever make it there

• What are the versions of mypy and Python you are using?
  mypy 0.720 with Python 3.7.3

• Do you see the same issue after installing mypy from Git master?
  Yes still exists on master

Commencez par la reproduction minimale en Python de l’issue et suivez le point d’entrée du rétrécissement de type fondé sur la vérité logique de mypy. Aucun fichier source ni test n’est indiqué dans le rapport ; le travail est terminé lorsque la branche conditionnelle réduit suffisamment Optional[int] pour que 100 / maybe_int passe la vérification de type, avec une couverture de régression.