feat: support empty arguments of ## operator

4 files changed, 70 insertions, 11 deletions

diff --git a/src/preprocess.c b/src/preprocess.c
index 268147e..54e3648 100644
--- a/src/preprocess.c
+++ b/src/preprocess.c
@@ -726,10 +726,23 @@ static int expand_macro(Preprocessor* pp, bool skip_newline, MacroExpansionConte
             int macro_param_idx = macro_find_param(macro, tok);
             if (macro_param_idx != -1) {
                 size_t arg_token_count = args->data[macro_param_idx].tokens.len;
-                replace_pp_tokens(pp, macro_name_pos + i + offset, macro_name_pos + i + offset + 1,
-                                  &args->data[macro_param_idx].tokens);
-                token_count += arg_token_count;
-                offset += arg_token_count - 1;
+                if (arg_token_count == 0) {
+                    // Empty argument: insert a placemarker token
+                    TokenArray placemarker_token;
+                    tokens_init(&placemarker_token, 1);
+                    Token* pm = tokens_push_new(&placemarker_token);
+                    pm->kind = TokenKind_placemarker;
+                    pm->loc = tok->loc;
+                    replace_pp_tokens(pp, macro_name_pos + i + offset, macro_name_pos + i + offset + 1,
+                                      &placemarker_token);
+                    token_count += 1;
+                    // offset stays the same (1 - 1 = 0)
+                } else {
+                    replace_pp_tokens(pp, macro_name_pos + i + offset, macro_name_pos + i + offset + 1,
+                                      &args->data[macro_param_idx].tokens);
+                    token_count += arg_token_count;
+                    offset += arg_token_count - 1;
+                }
             } else {
                 ++token_count;
             }
@@ -757,15 +770,35 @@ static int expand_macro(Preprocessor* pp, bool skip_newline, MacroExpansionConte
                     }
                 }
                 if (lhs_pos == -1 || rhs_pos == -1) {
-                    fatal_error("invalid usage of ## operator");
+                    fatal_error("%s:%d: invalid usage of ## operator", tok->loc.filename, tok->loc.line);
+                }
+
+                Token* lhs_tok = pp_token_at(pp, lhs_pos);
+                Token* rhs_tok = pp_token_at(pp, rhs_pos);
+                bool lhs_is_placemarker = lhs_tok->kind == TokenKind_placemarker;
+                bool rhs_is_placemarker = rhs_tok->kind == TokenKind_placemarker;
+
+                TokenArray result_tokens;
+                tokens_init(&result_tokens, 1);
+
+                if (lhs_is_placemarker && rhs_is_placemarker) {
+                    // Both are placemarkers: result is a placemarker
+                    Token* pm = tokens_push_new(&result_tokens);
+                    pm->kind = TokenKind_placemarker;
+                    pm->loc = tok->loc;
+                } else if (lhs_is_placemarker) {
+                    // Left is placemarker: result is the right token
+                    *tokens_push_new(&result_tokens) = *rhs_tok;
+                } else if (rhs_is_placemarker) {
+                    // Right is placemarker: result is the left token
+                    *tokens_push_new(&result_tokens) = *lhs_tok;
+                } else {
+                    // Neither is placemarker: concatenate them
+                    Token* concatenated = concat_two_tokens(lhs_tok, rhs_tok);
+                    *tokens_push_new(&result_tokens) = *concatenated;
                 }
-                Token* concatenated = concat_two_tokens(pp_token_at(pp, lhs_pos), pp_token_at(pp, rhs_pos));
 
-                // Replace the three tokens (lhs ## rhs) with the concatenated one
-                TokenArray single_token;
-                tokens_init(&single_token, 1);
-                *tokens_push_new(&single_token) = *concatenated;
-                replace_pp_tokens(pp, lhs_pos, rhs_pos + 1, &single_token);
+                replace_pp_tokens(pp, lhs_pos, rhs_pos + 1, &result_tokens);
                 token_count -= rhs_pos - lhs_pos;
                 i -= pos - lhs_pos;
                 token_count2 -= pos - lhs_pos - 1;
@@ -774,6 +807,14 @@ static int expand_macro(Preprocessor* pp, bool skip_newline, MacroExpansionConte
             }
         }
 
+        // Remove placemarker tokens after ## processing
+        for (size_t i = 0; i < token_count2; ++i) {
+            Token* tok = pp_token_at(pp, macro_name_pos + i);
+            if (tok->kind == TokenKind_placemarker) {
+                tok->kind = TokenKind_removed;
+            }
+        }
+
         // Inherit a source location from the original macro token.
         for (size_t i = 0; i < token_count2; ++i) {
             pp_token_at(pp, macro_name_pos + i)->loc = original_loc;
diff --git a/src/token.c b/src/token.c
index 7e54729..ebb803e 100644
--- a/src/token.c
+++ b/src/token.c
@@ -13,6 +13,8 @@ const char* token_kind_stringify(TokenKind k) {
         return "<whitespace>";
     else if (k == TokenKind_removed)
         return "<removed>";
+    else if (k == TokenKind_placemarker)
+        return "<placemarker>";
     else if (k == TokenKind_newline)
         return "<new-line>";
     else if (k == TokenKind_other)
diff --git a/src/token.h b/src/token.h
index 9dc89e8..e093a8c 100644
--- a/src/token.h
+++ b/src/token.h
@@ -12,6 +12,7 @@ typedef enum {
     TokenKind_hashhash,
     TokenKind_whitespace,
     TokenKind_removed,
+    TokenKind_placemarker,
     TokenKind_newline,
     TokenKind_other,
     TokenKind_character_constant,
diff --git a/tests/macro_operators.sh b/tests/macro_operators.sh
index a6d0445..57e8564 100644
--- a/tests/macro_operators.sh
+++ b/tests/macro_operators.sh
@@ -107,3 +107,18 @@ int main() {
     ASSERT_EQ(0, strcmp("FOO", BAZ));
 }
 EOF
+
+test_exit_code 0 <<'EOF'
+#include <helpers.h>
+
+#define CONCAT3(x, y, z) x##y##z
+#define FOO CONCAT3(, 2, 3)
+#define BAR CONCAT3(1, , 3)
+#define BAZ CONCAT3(1, 2, )
+
+int main() {
+    ASSERT_EQ(23, FOO);
+    ASSERT_EQ(13, BAR);
+    ASSERT_EQ(12, BAZ);
+}
+EOF