This adds an extension warning when using the preprocessor conditionals
in a language mode they're not officially supported in, and an opt-in
warning for compatibility with previous standards.
Fixes#55306
Differential Revision: https://reviews.llvm.org/D125178
WG14 adopted N2645 and WG21 EWG has accepted P2334 in principle (still
subject to full EWG vote + CWG review + plenary vote), which add
support for #elifdef as shorthand for #elif defined and #elifndef as
shorthand for #elif !defined. This patch adds support for the new
preprocessor directives.
More study has discovered this to not actually be useful: because
current C++20 implementations reject `#ifdef __VA_OPT__`, this can't
really be used as a feature-test mechanism. And it's not too hard to
detect __VA_OPT__ without this, for example:
#define THIRD_ARG(a, b, c, ...) c
#define HAS_VA_OPT(...) THIRD_ARG(__VA_OPT__(,), 1, 0, )
#if HAS_VA_OPT(?)
Partially reverts 0436ec2128.
These changes are intended to give code a path to move away from the GNU
,##__VA_ARGS__ extension, which is non-conforming in some situations and
which we'd like to disable in our conforming mode in those cases.
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.
llvm-svn: 315840
Ken Matsui