Support match statement in partially defined check

[ilinum]

This adds support for match statements for the partially-defined variables check. This should completely cover all match features.

In addition, during testing, I found a bug in the generic branch tracking logic, which is also fixed here.

Because match is only supported in python 3.10, I had to put the tests in a separate file, which is a bit unfortunate; let me know if you have better ideas.

[JukkaL]

This doesn't generate an error, though I think it should:

def f(x: int | str) -> int:
    match x:
        case int():
            y = 1
    return y  # no error

[ilinum]

The exact example you provided generates the error for me. Can you double check how you ran this?

I've added this exact example into the test.

[JukkaL]

Sorry, I was running the wrong code. The example works just fine. However, this related case seems to not work as expected:

def f(x: int | str) -> int:
    match x:
        case int():
            y = 1
        case str():
            y = 2
    return y  # error: Name "y" may be undefined

Since the match statement covers all the union items, there shouldn't be an error. Similarly, this generates a false positive:

def f2(x: int | str) -> int:
    if isinstance(x, int):
        y = 1
    elif isinstance(x, str):
        y = 2
    return y  # error: Name "y" may be undefined

However, mypy doesn't complain about a missing return statement in these related examples, so mypy can already detect if all union items are covered:

def f3(x: int | str) -> int:
    if isinstance(x, int):
        return 1
    elif isinstance(x, str):
        return 2

def f4(x: int | str) -> int:
    match x:
        case int():
            return 1
        case str():
            return 2

Since this also affects isinstance checks, this may not be directly related to this PR, and fixed in a separate PR.

[ilinum]

Good find!

Agreed, separate PR is probably a better place to do this. I created #13926.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉