Error "Cannot determine type of ..." of instance attribute with isinstance() check

[Athazo]

This is my very first time opening an Issue on GitHub, so don't be too harsh if I'm doing it wrong since I'm not really used to the conventions here ;)

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

• Please insert below the code you are checking with mypy

import numbers

class Foo():
    def __init__(self, x: int) -> None:
      self.x = x

    @property
    def x(self) -> int:
        return self._x

    @x.setter
    def x(self, value: int) -> None:
        if value >= 0 and isinstance(value, numbers.Integral):
            self._x = value

Edit: Simpler example, removed the property, still getting the same error:

import numbers

class Foo():
  def __init__(self, x: int) -> None:
    if x >= 0 and isinstance(x, numbers.Integral):
      self.x = x

• What is the actual behavior/output?
  Output: mypy_test.py:12: error: Cannot determine type of '_x'
  Everything works fine, if isinstance(value, numbers.Integral) is removed.

• What is the behavior/output you expect?
  No error

• What are the versions of mypy and Python you are using?
  mypy 0.701
  Python 3.7.2

• What are the mypy flags you are using? (For example --strict-optional)
  No flags at all.

[ilevkivskyi]

Hm, this is pretty bad. The problem is that mypy believes that int and numbers.Integral are non-overlapping, so the branch is unreachable. So essentially this is a duplicate of #3603 (already high priority).

See also #2636 for the discussion about numeric tower.