[red-knot] Consider all definitions after terminal statements unreachable

[dcreager]

We can use the new statically known branches feature to address #14014, by adding an "always false" visibility constraint immediately after each terminal statement.

Test Plan

The new mdtests failed (with incorrect reveal_type results, and spurious possibly-unresolved-reference errors) before adding the new visibility constraints.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[dcreager]

There are a couple of new diagnostics in the benchmark that don't look correct to me. I need to see if I can minimize that into an mdtest to diagnose.

[carljm]

Fantastic!! Love to see a feature that is easier than anticipated :)

[dylwil3]

I don't think you need to change anything for this PR, but just so it's on your radar: try/finally knows how to ruin any clean story. For example, the following test fails on this branch:

def f():
    x = 1
    while True:
        try:
            break
        finally:
            x = 2
    reveal_type(x)  # revealed: Literal[2] 

(it gives a revealed type of Literal[1] instead)

[carljm]

Yeah, we can handle finally in this PR or as a separate follow up PR, but it probably will need some special handling.

[dcreager]

For example, the following test fails on this branch:

Thanks @dylwil3! I added that as a failing test case. I'm going to poke at it briefly to see if it's easy to add for this PR

[dcreager]

Ooh, this is now incorrect. It should be Literal["before", "loop", "continue"]:

If i <= 0, the loop body won't execute, and x == "before".
If i > 0 and cond, the loop body will execute i times, each time assigning "loop".
If i > 0 and not cond, the loop body will execute i times, each time assigning "continue".

The continue statement should mark its flow as unreachable for when we join the if branches. But it should not be considered unreachable when we join with the for loop's continuation and exit paths.