Doxygen does not see the description of a pre-processor symbol

问题

Please do not mind the strangeness of the following minimal example (I would have to make it much larger to justify why I am doing things this way):

File test.cpp:

#include "a.h"

int main() {
  return 0;
}

File a.h:

namespace N { // without namespace all is well!
#include "b.h"
}

File b.h:

/// \file

/// Documentation of CMD
#define CMD 5

Doxygen 1.8.11 complains:

warning: Member CMD (macro definition) of file a.h is not documented.

Why doesn't Doxygen see the description of CMD and how do I fix this?

来源：https://stackoverflow.com/questions/40382849/doxygen-does-not-see-the-description-of-a-pre-processor-symbol