VC preprocessor bug?

Hi,

Please consider the following sample code (based on code found at
http://www.ddj.com/linux-open-source/184403745):

#include "stdafx.h"

struct Log
{
    Log& VARS_A;
    Log(): VARS_A(*this) {}
    void* Write() { return NULL; }
    template <typename T>
    Log& V(char const* n, T const& t) { return *this; }
};

Log log;

#define VARS log.VARS_A
#define VARS_A(v) VARS_A.V(#v, (v)).VARS_B
#define VARS_B(v) VARS_A.V(#v, (v)).VARS_A

#define LOG(p) p

int _tmain(int argc, _TCHAR* argv[])
{
    int x, y;
    void* r;

    r = VARS(x)(y).Write();
    // After preprocessing:
    // r = log.VARS_A.V("x", (x)).VARS_A.V("y", (y)).VARS_A.Write();

    r = LOG(VARS(x)(y).Write());
    // After preprocessing:
    // r = y;
    // Why? The only difference is that the whole stuff is enclosed in
LOG().

    return 0;
}

Is this a bug in the VC preprocessor, or am I doing something wrong?
Besides VC, I've only checked this with the online Comeau compiler
(http://www.comeaucomputing.com/tryitout/), and that compiled it just
fine.
If it is indeed a bug, can anyone suggest a workaround?

Thanks,

    Imre