A walrus if test is never settled, so its unreachable suite is not reported #5499

Describe the Bug

Describe the bug

Pyrefly reports a suite as unreachable when its if test is always false (or the else suite when the test is always true). It does not do this when the test is a walrus (x := value).

from typing import Literal

def falsy(value: Literal[False]) -> None:
    if value:
        print(1)  # This code is unreachable
    if x := value:
        print(2)  # no error

def truthy(value: Literal[True]) -> None:
    if x := value:
        print(3)
    else:
        print(4)  # no error

class AlwaysFalse:
    def __bool__(self) -> Literal[False]:
        return False

def user_defined(a: AlwaysFalse | None) -> None:
    if a and (b := a):
        print(5)  # This code is unreachable
    if b := (a and a):
        print(6)  # no error

Expected behavior

print(2), print(4) and print(6) are reported as unreachable.

Cause

declared_bool_value (pyrefly/lib/alt/solve.rs) computes the declared truth value of a branch test. It has no case for Expr::Named, so a walrus test falls to _ => None.

Proposed fix

A walrus has the value it assigns:

Expr::Named(named) => self.declared_bool_value(&named.value),

Sandbox Link

No response

(Only applicable for extension issues) IDE Information

No response