Fix: empty array doesn't override an array of tables in toml_merge

src/tomlc17.c

@@ -665,9 +665,13 @@ static int datum_copy(toml_datum_t *dst, toml_datum_t src, srcmap_t *sm,
   return -1;
 }
 
-// Check if datum is an array of tables.
+// Check if datum is an array of tables. An empty array is deliberately
+// excluded: it has no elements to type-check, so treating it as an array
+// of tables here would be true only vacuously, and datum_merge's caller
+// relies on this to distinguish "override with an empty array" from
+// "append zero table elements" (see below).
 static inline bool is_array_of_tables(toml_datum_t datum) {
-  bool ret = (datum.type == TOML_ARRAY);
+  bool ret = (datum.type == TOML_ARRAY) && (datum.u.arr.size > 0);
   for (int i = 0; ret && i < datum.u.arr.size; i++) {
     ret = (datum.u.arr.elem[i].type == TOML_TABLE);
   }

test/merge/test1.c

@@ -166,6 +166,33 @@ static void test_array_of_tables_overridden_by_plain_array() {
   check(doc1, doc2, expected);
 }
 
+// An array of tables overridden by an empty array must be replaced with
+// the empty array, not left untouched. is_array_of_tables() used to treat
+// an empty array as vacuously true (no elements fail the table check), so
+// merging in an empty array hit the append branch instead of the override
+// branch and appended zero elements onto the untouched destination.
+static void test_array_of_tables_overridden_by_empty_array() {
+  printf("Running test_array_of_tables_overridden_by_empty_array...\n");
+  const char *doc1 = "[[arr]]\n"
+                     "x = 1";
+  const char *doc2 = "arr = []";
+  const char *expected = "arr = []";
+  check(doc1, doc2, expected);
+}
+
+// The reverse direction, for completeness: an empty array overridden by a
+// real array of tables must end up with the array of tables, whether the
+// merge takes the append path or the override path.
+static void test_empty_array_overridden_by_array_of_tables() {
+  printf("Running test_empty_array_overridden_by_array_of_tables...\n");
+  const char *doc1 = "arr = []";
+  const char *doc2 = "[[arr]]\n"
+                     "x = 1";
+  const char *expected = "[[arr]]\n"
+                         "x = 1";
+  check(doc1, doc2, expected);
+}
+
 static void test_type_conflicts() {
   printf("Running test_type_conflicts...\n");
   const char *doc1 = "value = 42";
@@ -217,6 +244,8 @@ int main() {
   test_array_of_tables();
   test_plain_array_overridden_by_array_of_tables();
   test_array_of_tables_overridden_by_plain_array();
+  test_array_of_tables_overridden_by_empty_array();
+  test_empty_array_overridden_by_array_of_tables();
   test_type_conflicts();
   test_empty_documents();
   test_keys_outlive_inputs();