Narrow Union based on tuple fixed length check

[mthuurne]

When mypy 0.782 checks the following code:

from typing import Tuple, Union

def weird_cat(values: Union[Tuple[str, str], Tuple[str, str, str]]) -> str:
    if len(values) == 2:
        return values[0] + values[1]
    else:
        return values[0] + values[2] + values[1]

It reports:

testcase.py:7: error: Tuple index out of range  [misc]
            return values[0] + values[2] + values[1]
                               ^

There is no need to report an error here, since the tuple will always have a third element if the second return statement is reached.

I think mypy could filter the Union options using the len(values) == 2 guard, which is always true for Tuple[str, str] and always false for Tuple[str, str, str].

The example in this comment from @abey79 looks equivalent to this issue, but the test case for #8733 itself also includes a List, so it has to deal with runtime length rather than the static length of a tuple type.

[ilevkivskyi]

Duplicate of #1178