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

Beginne mit der minimalen Python-Reproduktion im Issue und verfolge den Einstiegspunkt für mypys wahrheitswertbasierte Typverengung. Im Bericht ist keine Quelldatei und kein Test genannt; abgeschlossen ist die Aufgabe, wenn der bedingte Zweig Optional[int] so weit verengt, dass 100 / maybe_int typgeprüft werden kann, mit Regressionstestabdeckung.