Documenting conditional exclusive code in Doxygen - c-preprocessor

Question

Consider
// EXTERNAL_MACRO is an external macro defined to some value by build system
#if EXTERNAL_MACRO == 1
# define EXCLUSIVE_MACRO_ONE
#elif EXTERNAL_MACRO == 2
# define EXCLUSIVE_MACRO_TWO
#else
# define EXCLUSIVE_MACRO_OTHER
#endif
At built time, only one of the EXCLUSIVE_MACRO_... macros is defined.
How can one document all three EXCLUSIVE_MACRO_... macros in Doxygen?
PREDEFINED configuration setting is not helpful for this because it allows to define EXTERNAL_MACRO to only a single value (and therefore document only a single EXCLUSIVE_MACRO_...).
This SO answer offers a work-around solution that does the job but it requires an additional logic. I am looking for a simpler answer, one that uses Doxygen configuration (if possible) rather than modifying the original code (though some modification is of course fine).

Answer 1

See related question and answer:
Document a config macro with doxygen
Basically:
In the doxygen configuration file, add:
PREDEFINED = IN_DOXYGEN
Somewhere in the source code, add:
/*
Exporting build configuration macros to doxygen,
so they get documented.
*/
#ifdef IN_DOXYGEN
# define EXCLUSIVE_MACRO_ONE
# define EXCLUSIVE_MACRO_TWO
# define EXCLUSIVE_MACRO_THREE
#endif /* IN_DOXYGEN */
Then the documentation for these macros is picked up properly.
If you don't want to change the source, you can add EXCLUSIVE_MACRO_ONE and friends in PREDEFINED, instead of setting EXTERNAL_MACRO.

Answer 2

Inspired by answer.
PROBLEM: We have an existing header that, based on an external macro (provided by build system), defines one of several exclusive macros during compilation. Since only one of the compile-time macros is defined (for each individual build), it is difficult to add Doxygen documentation to all of the exclusive options.
// DEFINED_EXTERNALLY is a macro provided by the build system (e.g. via -D). It is defined as 1, 2, or some other natural number.
// Based on DEFINED_EXTERNALLY we want to define one of several exclusive DEFINED_HERE_... macros (e.g. DEFINED_HERE_1, DEFINED_HERE_2, and DEFINED_HERE_OTHER).
// First, we check that none of the DEFINED_HERE_... macros is defined yet.
#if defined(DEFINED_HERE_1) || defined(DEFINED_HERE_2) || defined(DEFINED_HERE_OTHER)
# error "Conflict: macro DEFINED_HERE_... is already defined!"
#endif
// Then, we define one, and only one, of the DEFINED_HERE_... macros.
#if DEFINED_EXTERNALLY == 1
# define DEFINED_HERE_1
#elif DEFINED_EXTERNALLY == 2
# define DEFINED_HERE_2
#else
# define DEFINED_HERE_OTHER
#endif
SOLUTION: We temporarily define all variants of the DEFINED_HERE_... macros, together with their documentation. This will cause Doxygen to generate documentation for all of them. We then undefine all of the variants and resume normal macro definition logic.
As a result, we will have documentation for all variants yet only one of them will be defined during compilation as was the case before adding the documentation.
Original code
// DEFINED_EXTERNALLY is a macro provided by the build system (e.g. via -D). It is defined as 1, 2, or some other natural number.
// Based on DEFINED_EXTERNALLY we want to define one of several DEFINED_HERE_... macros (e.g. DEFINED_HERE_1, DEFINED_HERE_2, and DEFINED_HERE_OTHER).
// First, we check that none of the DEFINED_HERE_... macros is defined yet.
#if defined(DEFINED_HERE_1) || defined(DEFINED_HERE_2) || defined(DEFINED_HERE_OTHER)
# error "Conflict: macro DEFINED_HERE_... is already defined!"
#endif
New inserted code to generate documentation
/// \brief Value 1.
/// \details It does the following...
#define DEFINED_HERE_1
#undef DEFINED_HERE_1
/// \brief Value 2.
/// \details It does the following...
#define DEFINED_HERE_2
#undef DEFINED_HERE_2
/// \brief Other value.
/// \details It does the following...
#define DEFINED_HERE_OTHER
#undef DEFINED_HERE_OTHER
Original code
// Then, we define one, and only one, of the DEFINED_HERE_... macros.
#if DEFINED_EXTERNALLY == 1
# define DEFINED_HERE_1
#elif DEFINED_EXTERNALLY == 2
# define DEFINED_HERE_2
#else
# define DEFINED_HERE_OTHER
#endif
NB no change to the default Doxygen settings is required
ENABLE_PREPROCESSING = YES
MACRO_EXPANSION = NO
EXPAND_ONLY_PREDEF = NO
PREDEFINED =
EXPAND_AS_DEFINED =