[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.
llvm-svn: 315840

1 files changed, 15 insertions, 7 deletions

diff --git a/clang/lib/Lex/Preprocessor.cpp b/clang/lib/Lex/Preprocessor.cpp
index 1f9a469bc59..65df6a57f1d 100644
--- a/clang/lib/Lex/Preprocessor.cpp
+++ b/clang/lib/Lex/Preprocessor.cpp
@@ -121,12 +121,18 @@ Preprocessor::Preprocessor(std::shared_ptr<PreprocessorOptions> PPOpts,
 
   // We haven't read anything from the external source.
   ReadMacrosFromExternalSource = false;
-  
-  // "Poison" __VA_ARGS__, which can only appear in the expansion of a macro.
-  // This gets unpoisoned where it is allowed.
+
+  // "Poison" __VA_ARGS__, __VA_OPT__ which can only appear in the expansion of
+  // a macro. They get unpoisoned where it is allowed.
   (Ident__VA_ARGS__ = getIdentifierInfo("__VA_ARGS__"))->setIsPoisoned();
   SetPoisonReason(Ident__VA_ARGS__,diag::ext_pp_bad_vaargs_use);
-  
+  if (getLangOpts().CPlusPlus2a) {
+    (Ident__VA_OPT__ = getIdentifierInfo("__VA_OPT__"))->setIsPoisoned();
+    SetPoisonReason(Ident__VA_OPT__,diag::ext_pp_bad_vaopt_use);
+  } else {
+    Ident__VA_OPT__ = nullptr;
+  }
+
   // Initialize the pragma handlers.
   RegisterBuiltinPragmas();
   
@@ -667,13 +673,15 @@ bool Preprocessor::HandleIdentifier(Token &Identifier) {
   // unpoisoned it if we're defining a C99 macro.
   if (II.isOutOfDate()) {
     bool CurrentIsPoisoned = false;
-    if (&II == Ident__VA_ARGS__)
-      CurrentIsPoisoned = Ident__VA_ARGS__->isPoisoned();
+    const bool IsSpecialVariadicMacro =
+        &II == Ident__VA_ARGS__ || &II == Ident__VA_OPT__;
+    if (IsSpecialVariadicMacro)
+      CurrentIsPoisoned = II.isPoisoned();
 
     updateOutOfDateIdentifier(II);
     Identifier.setKind(II.getTokenID());
 
-    if (&II == Ident__VA_ARGS__)
+    if (IsSpecialVariadicMacro)
       II.setIsPoisoned(CurrentIsPoisoned);
   }