[GNU] Add ?: operator with omitted operand

parse.c

@@ -1936,7 +1936,7 @@ static Node *assign(Token **rest, Token *tok) {
   return node;
 }
 
-// conditional = logor ("?" expr ":" conditional)?
+// conditional = logor ("?" expr? ":" conditional)?
 static Node *conditional(Token **rest, Token *tok) {
   Node *cond = logor(&tok, tok);
 
@@ -1945,6 +1945,18 @@ static Node *conditional(Token **rest, Token *tok) {
     return cond;
   }
 
+  if (equal(tok->next, ":")) {
+    // [GNU] Compile `a ?: b` as `tmp = a, tmp ? tmp : b`.
+    add_type(cond);
+    Obj *var = new_lvar("", cond->ty);
+    Node *lhs = new_binary(ND_ASSIGN, new_var_node(var, tok), cond, tok);
+    Node *rhs = new_node(ND_COND, tok);
+    rhs->cond = new_var_node(var, tok);
+    rhs->then = new_var_node(var, tok);
+    rhs->els = conditional(rest, tok->next->next);
+    return new_binary(ND_COMMA, lhs, rhs, tok);
+  }
+
   Node *node = new_node(ND_COND, tok);
   node->cond = cond;
   node->then = expr(&tok, tok->next);

test/arith.c

@@ -130,6 +130,10 @@ int main() {
   ASSERT(-15, (char *)0xfffffffffffffff0 - (char *)0xffffffffffffffff);
   ASSERT(1, (void *)0xffffffffffffffff > (void *)0);
 
+  ASSERT(3, 3?:5);
+  ASSERT(5, 0?:5);
+  ASSERT(4, ({ int i = 3; ++i?:10; }));
+
   printf("OK\n");
   return 0;
 }