interface Box<T> {
    value: T
}

function isBox<T>(box: unknown): box is Box<T> {
    return true
}

const numberBox: Box<unknown> = {value:1}

if (isBox<number>(numberBox)) {
    numberBox.value
    //        ^?
}
if (isBox<any>(numberBox)) {
    numberBox.value
    //        ^?
}

if (isBox<any>(numberBox)) {
    numberBox.value
    //        ^? (property) Box<unknown>.value: unknown
}

if (isBox<any>(numberBox)) {
    numberBox.value
    //        ^? (property) Box<any>.value: any
}