Recursive macros via __VA_OPT__ - c++

Question

Is it legal to write recursive macros with __VA_OPT__?
GCC and Clang appear to not replace recursively, but I'm not sure if it's intentional (as __VA_OPT__ support is very recent).
C++ spec (§19.3.1/3: __VA_OPT__):
Otherwise, the replacement consists of the results of the expansion of
content as the replacement list of the current function-like macro
before rescanning and further replacement
Does the highlighted section above mean recursion isn't possible?
For example, to add a list of variadic macro parameters:
#define RECURSE(mFIRST, ...) + mFIRST __VA_OPT__(RECURSE(__VA_ARGS__))
int main(int argc, char const* const argv[])
{
return 1 RECURSE(2, 3, 4);
// Expected result: "return 1 + 2 + 3 + 4;"
}
Both GCC and Clang generate RECURSE in their post-preprocessing.
// NOTE: Below is the output from g++ -E
int main(int argc, char const* const argv[])
{
return 1 + 2 RECURSE(3, 4);
}
NOTE: If this is possible, more complex variadic macros could be written fairly easy, such as concatenate, as you can create a custom __VA_NO_OPT__ from __VA_OPT__, which lets you provide completely separate code for 1 and 2+ parameters.

Answer 1

The answer is yes! To the extent that you could already have recursive macros in C, you can do it in C++20 and __VA_OPT__ makes some things way, way nicer.
Here's an example (with explanation) of what you can do: define a FOR_EACH macro that applies a macro to a bunch of arguments and is way nicer than the horrible things you had to do prior to __VA_OPT__:
#define PARENS () // Note space before (), so object-like macro
#define EXPAND(arg) EXPAND1(EXPAND1(EXPAND1(EXPAND1(arg))))
#define EXPAND1(arg) EXPAND2(EXPAND2(EXPAND2(EXPAND2(arg))))
#define EXPAND2(arg) EXPAND3(EXPAND3(EXPAND3(EXPAND3(arg))))
#define EXPAND3(arg) EXPAND4(EXPAND4(EXPAND4(EXPAND4(arg))))
#define EXPAND4(arg) arg
#define FOR_EACH(macro, ...) \
__VA_OPT__(EXPAND(FOR_EACH_HELPER(macro, __VA_ARGS__)))
#define FOR_EACH_HELPER(macro, a1, ...) \
macro(a1) \
__VA_OPT__(FOR_EACH_AGAIN PARENS (macro, __VA_ARGS__))
#define FOR_EACH_AGAIN() FOR_EACH_HELPER
FOR_EACH(F, a, b, c, 1, 2, 3) // => F(a) F(b) F(c) F(1) F(2) F(3)