[red-knot] Failed to narrow in nested function

[dhruvmanila]

Summary

Red knot fails to narrow in the following code:

def foo(x: str | None):
    def inner():
        if x is not None:
            # Red knot is not narrowing here
            reveal_type(x)  # revealed: str | None

    return inner

Version

00e73dc

[mtshiba]

The current implementation of narrowing only covers cases where the predicate scope and the definition scope are the same. We need to be able to use predicates of different scopes.

This seems to duplicate some of the work I am doing in astral-sh/ty#164. Because when we want to do instance attribute narrowing, we use instance attribute definition and predicate of attribute access to the variable to which the instance is assigned, but the scopes of the two are generally different.

class C:
    def __init__(self):
        # definition
        self.x: int | None = None

c = C()
# predicate
if c.x is not None:
    reveal_type(c.x)  # int