Failure in type checking for `__new__` with mypy.

uint.py

#!/usr/bin/env python
# coding: utf-8

from typing import Optional

class Uint(object):
    def __new__(c, value: int) -> 'Optional[Uint]':
        if value < 0:
            return None
        else:
            return super().__new__(c)

    def __init__(self, value: int) -> None:
        self.__value = value

    @property
    def value(self) -> int:
        return self.__value


x = Uint(10) # An instance of `Uint`
y = Uint(-10) # None

print(x.value)
print(y.value) # This causes AttributeError, but mypy cannot detect.

uint_new.py

#!/usr/bin/env python
# coding: utf-8

from typing import Optional

class Uint(object):
    def __init__(self, value: int) -> None:
        if value < 0:
            raise ValueError

        self.__value = value

    @staticmethod
    def new(value: int) -> Optional[Uint]:
        try:
            return Uint(value)
        except:
            return None

    @property
    def value(self) -> int:
        return self.__value


x = Uint.new(10) # An instance of `Uint`
y = Uint.new(-10) # None

# print(x.value) # mypy: error
# print(y.value) # mypy: error

if x is not None: print(x.value)
if y is not None: print(y.value)

mypy --strict-optional --disallow-untyped-defs uint.py with mypy 0.4.4.

python/mypy#1020