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

Bắt đầu với bản tái hiện Python tối thiểu trong issue và lần theo điểm vào của việc thu hẹp kiểu dựa trên truthiness của mypy. Báo cáo không nêu tên tệp nguồn hay bài kiểm thử nào; được xem là hoàn thành khi nhánh điều kiện thu hẹp Optional[int] đủ để 100 / maybe_int vượt qua kiểm tra kiểu, kèm theo độ bao phủ hồi quy.