Preprocessor inconsistencies in Visual Studio - visual-studio

Question

I'm compiling the following C++ code in Visual Studio 2015 (update 3):
#include <iostream>
using namespace std;
////////////////////////////////////////
#define UNDERSCORE1(a,b) a ## _ ## b
#define UNDERSCORE(a,b) UNDERSCORE1(a,b)
#define STRINGIFY1(x) #x
#define STRINGIFY(x) STRINGIFY1(x)
#define VALUE(x) UNDERSCORE(x, VALUE)
#define NEXT(x) (VALUE(x) + 1)
/////////////////////////////////////////
#define X1_VALUE 0
#define X2_VALUE NEXT(X1)
#define X3_VALUE NEXT(X2)
#define TOTAL NEXT(X3)
int main() {
cout << STRINGIFY(TOTAL) << endl;
cout << TOTAL << endl;
return 0;
}
The result printed to stdout is very strange:
(X3_VALUE + 1)
3
When trying the same on gcc, build fails (expectedly).
When commenting out cout << TOTAL << endl; I get something different altogether:
(NEXT(X2) + 1)
Actually gcc behavior makes sense, since NEXT macro is called recursively: NEXT(X3) is expanded to X3_VALUE which in turn expands to NEXT(X2), so the second expansion of NEXT macro (NEXT(X2)) is not performed.
What doesn't make sense is Visual Studio behavior:
When printing the macro TOTAL using STRINGIFY, NEXT seems to be expanded twice to yield X3_VALUE.
When compiling the macro TOTAL directly to send it to cout, NEXT is expanded all the way! As if the preprocessor ran multiple times to recursively expand NEXT.
Another thing I tried was to compile this code in Visual Studio with the /P compiler option, to get the preprocessed code:
int main() {
cout << "(X3_VALUE + 1)" << endl;
cout << (((0 + 1) + 1) + 1) << endl;
return 0;
}
So is it, as I suspect, a bug in Visual Studio preprocessor? Or a legit undefined behavior?
Perhaps this behavior can be abused to truly expand macros recursively? I know that a limited recursion is possible with some tricks but is limited to a predefined number of scans. In this case I did not observe a limit to the number of times NEXT is expanded.

Answer 1

As Hans mentioned in the comments, the MSVC preprocessor is not conformant.
You can enable the conformant preprocessor with -experimental:preprocessor.
Here is a simplified repro + solution: https://godbolt.org/z/7u_-bH