[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.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@315840 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 14 insertions, 0 deletions

diff --git a/include/clang/Basic/DiagnosticLexKinds.td b/include/clang/Basic/DiagnosticLexKinds.td
index ec95dca6ac..6c35bab88a 100644
--- a/include/clang/Basic/DiagnosticLexKinds.td
+++ b/include/clang/Basic/DiagnosticLexKinds.td
@@ -346,6 +346,20 @@ def ext_pp_extra_tokens_at_eol : ExtWarn<
 def ext_pp_comma_expr : Extension<"comma operator in operand of #if">;
 def ext_pp_bad_vaargs_use : Extension<
   "__VA_ARGS__ can only appear in the expansion of a C99 variadic macro">;
+
+def ext_pp_bad_vaopt_use : Extension<
+  "__VA_OPT__ can only appear in the expansion of a variadic macro">;
+def err_pp_missing_lparen_in_vaopt_use : Error<
+  "missing '(' following __VA_OPT__">;
+def err_pp_vaopt_nested_use : Error<
+  "__VA_OPT__ cannot be nested within its own replacement tokens">;
+
+def err_vaopt_paste_at_start : Error<
+  "'##' cannot appear at start of __VA_OPT__ argument">;
+
+def err_vaopt_paste_at_end
+    : Error<"'##' cannot appear at end of __VA_OPT__ argument">;
+
 def ext_pp_macro_redef : ExtWarn<"%0 macro redefined">, InGroup<MacroRedefined>;
 def ext_variadic_macro : Extension<"variadic macros are a C99 feature">,
   InGroup<VariadicMacros>;