Strange stateful behavior with inferred type predicate type #17451

function tryCast<T, U extends T>(value: T, predicate: (t: T) => t is U): U | undefined {
    return predicate(value) ? value : undefined;
}

declare const sn: string | number;

const x: number | undefined = tryCast(sn, (n): n is number => typeof n === "number"); // Always works

const y = tryCast(sn, (n): n is number => typeof n === "number");

const z: number | undefined = y;