[c++2a] Implement P0306 VA_OPT (Comma omission and comma deletion)
This patch implements an extension to the preprocessor:
VA_OPT(contents) --> which expands into its contents if variadic arguments are supplied to the parent macro, or behaves as an empty token if none.
- Currently this feature is only enabled for C++2a (this could be enabled, with some careful tweaks, for other dialects with the appropriate extension or compatibility warnings)
- The patch was reviewed here: https://reviews.llvm.org/D35782 and asides from the above (and moving some of the definition and expansion recognition logic into the corresponding state machines), I believe I incorporated all of Richard's suggestions.
A few technicalities (most of which were clarified through private correspondence between rsmith, hubert and thomas) are worth mentioning. Given:
#define F(a,...) a #__VA_OPT__(a ## a) a ## __VA_OPT__(__VA_ARGS__) - The call F(,) Does not supply any tokens for the variadic arguments and hence VA_OPT behaves as a placeholder. - When expanding VA_OPT (for e.g. F(,1) token pasting occurs eagerly within its contents if the contents need to be stringified. - A hash or a hashhash prior to VA_OPT does not inhibit expansion of arguments if they are the first token within VA_OPT. - When a variadic argument is supplied, argument substitution occurs within the contents as does stringification - and these resulting tokens are inserted back into the macro expansions token stream just prior to the entire stream being rescanned and concatenated.
See wg21.link/P0306 for further details on the feature.
Acknowledgment: This patch would have been poorer if not for Richard Smith's usual thoughtful analysis and feedback.