diff --git a/sigc++/functors/slot.h b/sigc++/functors/slot.h
index 3f4b510..fc7f2af 100644
--- a/sigc++/functors/slot.h
+++ b/sigc++/functors/slot.h
@@ -194,7 +194,9 @@ struct slot_call
  */
 #ifndef DOXYGEN_SHOULD_SKIP_THIS
 template<typename T_return, typename... T_arg>
-class slot;
+class slot final {
+  static_assert(sizeof...(T_arg) < 0, "The slot<R, T...> syntax has been removed. Use the slot<R(T...)> syntax.");
+};
 #endif // DOXYGEN_SHOULD_SKIP_THIS
 
 template<typename T_return, typename... T_arg>
diff --git a/sigc++/signal.h b/sigc++/signal.h
index 2075bc3..675c1a0 100644
--- a/sigc++/signal.h
+++ b/sigc++/signal.h
@@ -581,7 +581,9 @@ class signal_with_accumulator : public signal_base
  */
 #ifndef DOXYGEN_SHOULD_SKIP_THIS
 template<typename T_return, typename... T_arg>
-class signal;
+class signal final {
+  static_assert(sizeof...(T_arg) < 0, "The signal<R, T...> syntax has been removed. Use the signal<R(T...)> syntax.");
+};
 #endif // DOXYGEN_SHOULD_SKIP_THIS
 
 template<typename T_return, typename... T_arg>