> If a programming language can tell when a variable goes out of scope, or its lifetime ends

Whether a variable goes out of scope is trivial in many languages. The problem is with determining whether the lifetime of a value ends.

For example:

    func foo(items, moreItems) {
        b = new bar()
        if coinflip() {
            items.append(b)
        }
        if coinflip() {
            moreItems.append(b)
        }
        // ‘b’ goes out of scope here, but its value
        // may live on inside ‘items’ and/or moreItems
        // and will have to be destroyed when it’s no
        // longer stored in either.
    }

In general, once you allow for dynamic allocation and references that can be copied (so that multiple objects ‘know’ of the allocated object), it can be very difficult to determine exactly when the last reference to such an object ceases to exist.