Attempting to compile this small example program with gcc, with
SOME_MACRO defined on the command line:
#if !defined(SOME_MACRO)
#define MACRO_1() 1
#else
#define MACRO_1()
#endif
#if defined(SOME_MACRO)
int main() { return 0; }
#elif MACRO_1() == 1
int main() { return 1; }
#endif
gives the error:
eliftest.cpp:9:17: error: operator '==' has no left operand
"g++" -ftemplate-depth-128 -O0 -fno-inline -Wall -pedantic -g
-Wno-variadic-macros -DSOME_MACRO -c "eliftest.cpp"
If SOME_MACRO is defined, it seems to me that gcc should not be
expanding the expression 'MACRO_1() == 1' and reporting an error.
If the program above is:
#if !defined(SOME_MACRO)
#define MACRO_1() 1
#else
#define MACRO_1()
#endif
#if defined(SOME_MACRO)
int main() { return 0; }
#else
#if MACRO_1() == 1
int main() { return 1; }
#endif
#endif
no compiler error is reported. This suggests that #elif is not the
shorthand equivalent to #else-#if-#endif that one would expect in C++.
