False positive `used-before-assignment` for type annotations that take assignment via nonlocal

[IRDonch]

Bug description

Create test.py with the following content:

def f():
    x: int

    def g():
        nonlocal x
        x = 5

    g()
    print(x)

f()

Configuration

No response

Command used

pylint test.py

Pylint output

************* Module test
test.py:9:10: E0602: Undefined variable 'x' (undefined-variable)

------------------------------------------------------------------
Your code has been rated at 3.75/10 (previous run: 3.75/10, +0.00)

Expected behavior

There should be no errors reported.

Pylint version

pylint 2.12.1
astroid 2.9.0
Python 3.6.9 (default, Jan 26 2021, 15:33:00)
[GCC 8.4.0]

OS / Environment

No response

Additional dependencies

No response

[IRDonch]

This is a regression in pylint 2.12. Probably caused by #5158.

[DanielNoord]

This might be difficult to solve as it requires control-flow knowledge. We can't do a simple check for a definition in an inner function as then this wouldn't be catched:

def f():
    x: int

    def g():
        nonlocal x
        x = 5

    print(x) # x is undefined
    g()

f()

For now I would advise to add an disable to this line. Sorry about that!

[IRDonch]

I understand that it's undecidable in the general case. You could, however, err on the side of avoiding false positives by only emitting this error if the variable is not assigned in any scope.

[DanielNoord]

I don't remember the code by heart but I'll have a look in the near future. I'm not sure whether that approach would greatly increase the amount of false negatives, but I/we can at least try.

If somebody else would like to have a look before me, please go ahead!

[jacobtylerwalls]

On 2.13.x this emits used-before-assignment instead due to #5718