No error when attempting to use property object as that property's type

• Are you reporting a bug, or opening a feature request? 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 NamedTuple


class C(NamedTuple):
    a: int = 1


print(C.a * 2)

• What is the actual behavior/output?

mypy a.py produces no output.

• What is the behavior/output you expect?

An error at C.a * 2, as it will cause an error at runtime: TypeError: unsupported operand type(s) for *: 'property' and 'int'

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

python --version: 3.7.2
mypy --version: 0.660+dev.97d7760dca6d95806a97b02992ae88a082898a0e

Do you see the same issue after installing mypy from Git master? Yes

• What are the mypy flags you are using? (For example --strict-optional) None

まず、報告された例を使って mypy a.py を実行し、出力がない一方で Python は指定された TypeError を発生させることを確認します。mypy が NamedTuple のプロパティアクセスと算術式をどのように処理するかを追跡します。このケースで mypy が C.a * 2 に対してエラーを報告すれば完了です。