pyrefly disagrees with mypy & pyright on type of captured variable in nested function

[jepler]

Describe the Bug

This is another false positive adapted and minimized from my project https://github.com/jepler/wwvbpy which type checks without errors under mypy --strict (and a WIP version also type checks without errors in pyright)

Broadly, and with my guessing hat on, the problem seems to be that the inferred type of the captured table_start has not been narrowed by assert table_start is not None in the outer scope.

from typing import TYPE_CHECKING

def f(rows: list[int]) -> int:
    table_start: int|None = None
    for r in rows:
        if table_start is None:
            table_start = r

    assert table_start is not None

    def inner() -> int:
        return table_start - 1

    return inner()

print(f([2024]))

Sandbox Link

https://pyrefly.org/sandbox/?code=GYJw9gtgBALgngBwJYDsDmUkQWEMoAqAmgAoCiA+gMIASZVA0gJIByA4gFAcAmApsFGAAKcAHcAzgC4oAGyTiYAbVQwAugEooAWgB8mFDEkcoJ2AEMARjN4UFZvNJUAfFmBS8oAXiiv3x08C4UCD6wWASRqZRmAIwlta2cXiY4j5uvJHRWXFWNnbJ3iBcUWbi4rzJOQn5+PJQKGD4vrzFpnwCqO4gQpq6+ob+WSC8MACuICjmuYn2+FpQAIytJsNjE-pdPVwIICpCwooATAAMhwAsGupAA

(Only applicable for extension issues) IDE Information

No response

[ndmitchell]

Thanks for the report! A simpler example: https://pyrefly.org/sandbox/?code=CYUwZgBGAUAeBcECWA7ALhAPhAcgexRAEp4AoCCiAQwGcaQAnDWZGiFPDfQ08y0SKkINoRCAFoAfMnRlK8iAxBoArgxQRYveUMajSQA

The issue here is that when we have a captured variable in a nested function we use the Anywhere binding for it, which is the union of all possible values at all points. However, that's a bit pessimistic - we could use the union of all values after it the nested function.

[jepler]

yes, I agree that's probably getting at the same root.

[yangdanny97]

This looks similar to #40