fix PR7943, a corner case with the GNU __VA_ARGS__ comma

swallowing extension.

llvm-svn: 111701

clang/lib/Lex/TokenLexer.cpp

@@ -268,6 +268,13 @@ void TokenLexer::ExpandFunctionArguments() {
       // Remove the paste operator, report use of the extension.
       PP.Diag(ResultToks.back().getLocation(), diag::ext_paste_comma);
       ResultToks.pop_back();
+      
+      // If the comma was right after another paste (e.g. "X##,##__VA_ARGS__"),
+      // then removal of the comma should produce a placemarker token (in C99
+      // terms) which we model by popping off the previous ##, giving us a plain
+      // "X" when __VA_ARGS__ is empty.
+      if (!ResultToks.empty() && ResultToks.back().is(tok::hashhash))
+        ResultToks.pop_back();
     }
     continue;
   }

clang/test/Preprocessor/macro_fn_comma_swallow.c

@@ -19,3 +19,8 @@ X3(foo)
 // PR3880
 #define X4(...)  AA , ## __VA_ARGS__ BB
 X4()
+
+// RUN: %clang_cc1 %s -E | grep '5: 1'
+// PR7943
+#define X5(x,...) x##,##__VA_ARGS__
+5: X5(1)