Bad operand type for unary -: NoneType

[fepegar]

Steps to reproduce

Run pylint on a file with this code:

"""x"""
a = None
if a is None:
    a = 1
b = -a

Current behavior

Get error bad operand type for unary -: NoneType at line 5.

Expected behavior

Get no errors, since a is always == 1.

pylint --version output

pylint 1.7.4,
astroid 1.5.3
Python 3.6.3 |Anaconda custom (64-bit)| (default, Nov  8 2017, 18:10:31)
[GCC 4.2.1 Compatible Clang 4.0.1 (tags/RELEASE_401/final)]

[PhilippSelenium]

Looks like a duplicate of #1472

[Pierre-Sassoulas]

You're right @PhilippSelenium. We have a few dozen of issues concerning control-flow, but this one is an exact match.