--warn-unused-ignore inconsistent with removed branches

[dvarrazzo]

Bug Report

I seems to understand that mypy can detect unused branched (ones checked against sys.version_info) and avoid checking them. However --warn-unused-ignore doesn't apply the same filtering.

As a consequence, an # ignore in a branch that goes unchecked is reported as unused, instead of being ignored. So you can have a statement that is reported as error running mypy with a certain Python version, but, if ignored, the same line will be reported as unneeded with a different Python version.

To Reproduce

# test.py
import sys

def f(a: str) -> int:
    return len(a)

f("hello")
if sys.version_info < (3, 7):
    f(b"world")

Running mypy --warn-unused-ignores test.py with Python 3.6, it will report an arg-type error on the last line, whereas a Python 3.8 test run will be clean.

If a type: ignore[arg-type] comment is added to the last line, the Python 3.6 run will be clean, but Python 3.8 will fail with "unused 'type: ignore' comment".

The expectation is that the type: ignore would be ignored by Python 3.8, if such branch is not checked.

Your Environment

Tested with mypy 0.782, 0.790

[hauntsaninja]

I think this is a duplicate of #8823 , you can use cast as a workaround