The new __VA_OPT__ feature in C23

C23 brings a new macro feature that eases programming with variadic macros a lot. Before it has been a challenge to detect if a ... argument was actually empty or contained at least one real token. This feature was introduced into C23 by a paper by Thomas Köppe “Comma omission and comma deletion” A similar feature had already been integrated to C++20 before.

As an example for the feature, suppose we want to augment calls to fprintf such that two types of problems are detected:

1. If there is only a format argument and no others, we want to use fputs to avoid scanning the format at execution time.
2. If there are more than one arguments, it should be assured that the format is a string literal, such that the contents can be parsed at compile time.