WHY THE FUCK DOES THIS COMPILE?

gistfile1.txt

public class Overflow
{
    public static void main(String[] args)
    {
        int value = 2000000000;

        long increment = 1000000000;

        value += increment;   // no problemo!

        System.out.println(value);
    }
}

By the way,

value = value + increment;    // compiler error