Mapping generic types breaks inference/narrowing #16267

interface I {
    x: number | null;
}

function generic<T extends I>(a: T, b: Partial<T>) {
    if (a.x != null) {
        a.x.toFixed(2); // OK
    }

    if (b.x != null) {
        b.x.toFixed(2); // ERROR: Property 'toFixed' does not exist on type 'T["x"]'
    }
}

function notGeneric(a: I, b: Partial<I>) {
    if (a.x != null) {
        a.x.toFixed(2); // OK
    }

    if (b.x != null) {
        b.x.toFixed(2); // OK
    }
}

class Component<P, S> {
    props: Readonly<{ children?: ReactNode }> & Readonly<P>;
    // snip
}