Tuple index out of range doesn't take into account len check when accessing Union of Tuples of varying lengths

[posita]

Where a type is a Union of Tuples of varying lengths, Mypy remains blissfully unaware of any len validation before accessing elements beyond the minimum length.

# test_case.py
from typing import Optional, Tuple, Union

def get_third_thing_works_correctly(thing: Tuple[int, int, Optional[int]]):
    return thing[2]

def get_third_thing_errors_correctly(thing: Tuple[int, int]):
    return thing[2]  # <-- correctly results in "Tuple index out of range [misc]"

def get_third_thing_false_positive(
    thing: Union[Tuple[int, int], Tuple[int, int, Optional[int]]]
):
    if len(thing) >= 3:
        return thing[2]  # <-- false positive
    else:
        return None

Results:

% python --version
Python 3.10.6
% mypy --version
mypy 0.982 (compiled: yes)
% mypy --config=/dev/null test_case.py
/dev/null: No [mypy] section in config file
test_case.py:8: error: Tuple index out of range
test_case.py:14: error: Tuple index out of range
Found 2 errors in 1 file (checked 1 source file)

[AlexWaygood]

This is arguably a duplicate of #1178, though it's a slightly different sympton of the same problem

[JelleZijlstra]

It's the same thing: a feature request that we use len() for type narrowing. Closing as a duplicate.