It's a common misconception that in C, whitespace doesn't matter. I can't imagine somebody hasn't come up with this in GNU C:

    #include <stdio.h>
    #define CAT_IMPL(c, d) (c ## d)
    #define CAT(c, d) CAT_IMPL(c, d)
    #define x CAT(a, __LINE__)

    int main()
    {
        int a9 = 2, a10 = 0;
        printf("%d\n", x ==
            x + 2);
        return 0;
    }

Prints `1`.