Cannot detect when function returns None but should return int

[hvnsweeting]

• 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.

# mypytrueneg.py
def hello2():
    # type: () -> int
    return None


def hello():
    # type: () -> None
    return 1

• What is the actual behavior/output?

 $ mypy mypytrueneg.py
mypytrueneg.py:8: error: No return value expected

• What is the behavior/output you expect?
  mypy should detect that hello2 returns None and it is not int as statically type declared.

• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?

$ python --version
Python 3.5.2

$ mypy --version
mypy 0.560

Also tried
$ mypy --version
mypy 0.570-dev-b0af822a79465a93d55e67b5aea74551a63c08f4

[JukkaL]

By default, None is considered compatible with everything. You can run mypy with --strict-optional and it will catch the error.