Implement GCC extended semantics of __VA_ARGS__

Author: robertoraggi

src/parser/cxx/preprocessor.cc

@@ -2355,6 +2355,22 @@ auto Preprocessor::Private::substitute(TokList *pointOfSubstitution,
       continue;
     }
 
+    if (lookat(ts, TokenKind::T_COMMA, TokenKind::T_HASH_HASH, "__VA_ARGS__")) {
+      auto comma = ts->tok;
+      ts = ts->next->next;
+
+      if (auto actual = lookupMacroArgument(ts, macro, actuals)) {
+        auto [startArg, endArg] = *actual;
+        if (startArg != endArg) {
+          appendToken(comma);
+          for (auto it = startArg; it != endArg; ++it) {
+            appendToken(&*it);
+          }
+        }
+        continue;
+      }
+    }
+
     if (lookat(ts, TokenKind::T_IDENTIFIER, TokenKind::T_HASH_HASH)) {
       if (auto actual = lookupMacroArgument(ts, macro, actuals)) {
         auto [startArg, endArg] = *actual;

tests/unit_tests/preprocessor/va_args_003.cc

@@ -0,0 +1,10 @@
+// RUN: %cxx -E -P %s -o - | %filecheck %s
+
+#define X(a, ...) static_assert(a, ## __VA_ARGS__)
+
+static_assert(true);
+static_assert(true, "it must be true");
+
+//      CHECK: static_assert(true);
+// CHECK-NEXT: static_assert(true, "it must be true");
+