How to implement mutable optional in Kotlin?

The compiler wants you to write code that will be type-safe for all possible T, both nullable and not-null (unless you specify a not-null upper bound for the type parameter, such as T : Any, but this is not what you need here).

If you store T? in a property, it is a different type from T in case of not-null type arguments, so you are not allowed to use T and T? interchangeably.

However, making an unchecked cast allows you to bypass the restriction and return the T? value as T. Unlike the not-null assertion (!!), the cast is not checked at runtime, and it won't fail when it encounters a null.

Change the get() function as follows:

fun get(): T {
    if (!isSet) {
        throw Error("Value not set")
    }
    @Suppress("unchecked_cast")
    return value as T
}