slot|signal: static_assert not using R,T... syntax

This can lead to clearer errors by explaining the user's error, not just
saying they used an incomplete type (why is it incomplete?). I don't use
only static_assert(false) because that's ill-formed before C++23, AFAIK,
& even if it's OK in some cases I don't grok which... so hope this works

libsigcplusplus#86

Author: db-src

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>

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>