pylint tracks the wrong return type of an overloaded function

[ziyuang]

Bug description

from typing import overload


@overload
def f(x: int, y: int) -> tuple[int, int]:
    ...


@overload
def f(x: int, y: int, z: int) -> tuple[int, int, int]:
    ...


def f(x: int, y: int, z: int | None = None) -> tuple[int, int] | tuple[int, int, int]:
    return (x, y) if z is None else (x, y, z)


def main():
    _, _ = f(1, 2)
    _, _, _ = f(1, 2, 3)

Configuration

Command used

pylint a.py

Pylint output

************* Module a
a.py:20: W0632 Possible unbalanced tuple unpacking with sequence defined at line 15: left side has 3 labels, right side has 2 values

Expected behavior

No error.

Pylint version

pylint 3.3.8
astroid 3.3.11
Python 3.10.9 (main, May  9 2024, 07:44:19) [GCC 13.2.0]

OS / Environment

$ uname -a
Linux <hostname> 6.8.0-86-generic #87-Ubuntu SMP PREEMPT_DYNAMIC Mon Sep 22 18:03:36 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux

Additional dependencies

[mbyrnepr2]

Thanks for the report.
It doesn't seem to have anything to do with overloaded functions; I see the same result if the decorated functions are removed from the example.
I feel this is in the same vein as #5671 and happening because the if z is None else is not taken into account.