Do not generate empty fetch functions and empty constructors.

GitOrigin-RevId: 65f0f7e37f5af8d6e256b6c7b7c07241354b7b45

td/generate/tl_writer_cpp.cpp

@@ -517,15 +517,21 @@ std::string TD_TL_writer_cpp::gen_fetch_function_begin(const std::string &parser
   assert(arity == 0);
 
   if (parser_type == 0) {
-    return "\n" + returned_type + class_name + "::fetch(" + parser_name +
-           " &p) {\n"
-           "  return make_tl_object<" +
-           class_name +
-           ">(p);\n"
-           "}\n\n" +
-           class_name + "::" + class_name + "(" + parser_name +
-           " &p)\n"
-           "#define FAIL(error) p.set_error(error)\n";
+    std::string result = "\n" + returned_type + class_name + "::fetch(" + parser_name +
+                         " &p) {\n"
+                         "  return make_tl_object<" +
+                         class_name + ">(";
+    if (field_num == 0) {
+      result += ");\n";
+    } else {
+      result +=
+          "p);\n"
+          "}\n\n" +
+          class_name + "::" + class_name + "(" + parser_name +
+          " &p)\n"
+          "#define FAIL(error) p.set_error(error)\n";
+    }
+    return result;
   }
 
   return "\n" + returned_type + class_name + "::fetch(" + parser_name +
@@ -542,9 +548,11 @@ std::string TD_TL_writer_cpp::gen_fetch_function_end(bool has_parent, int field_
   }
 
   if (parser_type == 0) {
+    if (field_num == 0) {
+      return "}\n";
+    }
     return "#undef FAIL\n"
-           "{" +
-           (field_num == 0 ? "\n  (void)p;\n" : std::string()) + "}\n";
+           "{}\n";
   }
 
   if (parser_type == -1) {

td/generate/tl_writer_h.cpp

@@ -258,10 +258,15 @@ std::string TD_TL_writer_h::gen_fetch_function_begin(const std::string &parser_n
   std::string returned_type = "object_ptr<" + parent_class_name + "> ";
 
   if (parser_type == 0) {
-    return "\n"
-           "  static " +
-           returned_type + "fetch(" + parser_name + " &p);\n\n" + "  explicit " + class_name + "(" + parser_name +
-           " &p);\n";
+    std::string result =
+        "\n"
+        "  static " +
+        returned_type + "fetch(" + parser_name + " &p);\n";
+    if (field_num != 0) {
+      result += "\n"
+                "  explicit " + class_name + "(" + parser_name + " &p);\n";
+    }
+    return result;
   }
 
   assert(arity == 0);

td/generate/tl_writer_td.cpp

@@ -50,7 +50,7 @@ bool TD_TL_writer::is_combinator_supported(const tl::tl_combinator *constructor)
 }
 
 bool TD_TL_writer::is_default_constructor_generated(const tl::tl_combinator *t) const {
-  return tl_name == "td_api" || t->var_count > 0 || t->args.empty();
+  return tl_name == "td_api" || t->var_count > 0;
 }
 
 int TD_TL_writer::get_storer_type(const tl::tl_combinator *t, const std::string &storer_name) const {