Regression: used-before-assignment error not reported for function arguments

[pbasista]

Steps to reproduce

Run Pylint on the following code:

"""A module to demonstrate Python's UnboundLocalError exception."""

def test(tmp=0):
    """Demonstrate UnboundLocalError exception on a function argument."""
    print(tmp)
    def inner():
        """Python considers "tmp" as a local variable in this function."""
        print(tmp)  # UnboundLocalError is raised here
        tmp = 1
    inner()
    print(tmp)


test()

Current behavior:

No warning or error is reported.

However, the code fails to run because of this well-known issue with using variables considered as local before their assignment. The following exception is raised:

UnboundLocalError: local variable 'tmp' referenced before assignment

Expected behavior

used-before-assignment error is reported on line where tmp is first used in function inner.

This error is correctly reported by Pylint 1.9.3, but not by Pylint versions 2 and above.

A similar error is correctly reported by Pylint versions 2 and above when the tmp variable is locally defined within the outer function, but not when it is a function's argument.

pylint --version output

pylint 2.1.0
astroid 2.0.2
Python 3.6.6 (default, Jun 27 2018, 14:44:17)
[GCC 8.1.0]
(the current master branch version 7be00a9)

[PCManticore]

Thanks for the report, I can also reproduce this regression.