Improper inference via length for a union of tuples

[Fulguritude]

Bug Report

mypy incorrectly infers a Tuple index out of range error when discriminating between two tuples in a union via their length, even when using a literal int when analyzing the length of the tuple instance.

To Reproduce

from typing import Tuple, Union

Tuple3 = Tuple[str, str, str]
Tuple4 = Tuple[str, str, str, str]
Tuples = Union[Tuple3, Tuple4]

def test_tuples(my_tuple: Tuples) -> None:
    if len(my_tuple) >= 4:
        my_tuple[3]  # misc - Tuple index out of range

Expected Behavior
Not returning an "index out of range" error, when it is clearly impossible for it to be the case.

Actual Behavior
mypy infers misc - Tuple index out of range

Your Environment

• Mypy version used: 0.950 (compiled: yes)
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.10
• Operating system and version: pop-os 22.04

[JelleZijlstra]

Duplicate of #1178