From 083c27559e5d8fce9c3b588fc4c01769ca9dd10d Mon Sep 17 00:00:00 2001
From: Rui Ueyama <ruiu@cs.stanford.edu>
Date: Thu, 23 Jul 2020 14:28:33 +0900
Subject: [PATCH] [GNU] Handle ,##__VA_ARG__

---
 preprocess.c | 16 ++++++++++++++++
 test/macro.c |  8 ++++++++
 2 files changed, 24 insertions(+)

diff --git a/preprocess.c b/preprocess.c
index d0cf4d3..049e6a4 100644
--- a/preprocess.c
+++ b/preprocess.c
@@ -526,6 +526,22 @@ static Token *subst(Token *tok, MacroArg *args) {
       continue;
     }
 
+    // [GNU] If __VA_ARG__ is empty, `,##__VA_ARGS__` is expanded
+    // to the empty token list. Otherwise, its expaned to `,` and
+    // __VA_ARGS__.
+    if (equal(tok, ",") && equal(tok->next, "##")) {
+      MacroArg *arg = find_arg(args, tok->next->next);
+      if (arg && !strcmp(arg->name, "__VA_ARGS__")) {
+        if (arg->tok->kind == TK_EOF) {
+          tok = tok->next->next->next;
+        } else {
+          cur = cur->next = copy_token(tok);
+          tok = tok->next->next;
+        }
+        continue;
+      }
+    }
+
     if (equal(tok, "##")) {
       if (cur == &head)
         error_tok(tok, "'##' cannot appear at start of macro expansion");
diff --git a/test/macro.c b/test/macro.c
index a088984..7f8b926 100644
--- a/test/macro.c
+++ b/test/macro.c
@@ -382,6 +382,14 @@ int main() {
   ASSERT(0, ({ char buf[100]; M30(buf, "foo%d", 3); strcmp(buf, "foo3"); }));
   ASSERT(0, ({ char buf[100]; M30(buf, "foo%d%d", 3, 5); strcmp(buf, "foo35"); }));
 
+#define M31(buf, fmt, ...) sprintf(buf, fmt, ## __VA_ARGS__)
+  ASSERT(0, ({ char buf[100]; M31(buf, "foo"); strcmp(buf, "foo"); }));
+  ASSERT(0, ({ char buf[100]; M31(buf, "foo%d", 3); strcmp(buf, "foo3"); }));
+  ASSERT(0, ({ char buf[100]; M31(buf, "foo%d%d", 3, 5); strcmp(buf, "foo35"); }));
+
+#define M31(x, y) (1, ##x y)
+  ASSERT(3, M31(, 3));
+
   printf("OK\n");
   return 0;
 }