fix: allow final classes as SM deps and SM classes (#483)

include/boost/sml.hpp

@@ -181,8 +181,11 @@ template <class T, class U>
 struct is_empty_base : T {
   U _;
 };
-template <class T>
+template <class T, class = void>
 struct is_empty : aux::integral_constant<bool, sizeof(is_empty_base<T, none_type>) == sizeof(none_type)> {};
+// Specialisation: final classes cannot be used as base; treat them as non-empty. (#483)
+template <class T>
+struct is_empty<T, aux::enable_if_t<bool(__is_final(T))>> : aux::false_type {};
 template <class>
 struct function_traits;
 template <class R, class... TArgs>
@@ -1693,7 +1696,8 @@ struct should_not_instantiate_statemachine_class
 
 template <class Tsm>
 struct should_not_subclass_statemachine_class
-    : integral_constant<bool, is_empty<typename Tsm::sm>::value || should_not_instantiate_statemachine_class<Tsm>::value> {};
+    : integral_constant<bool, is_empty<typename Tsm::sm>::value || should_not_instantiate_statemachine_class<Tsm>::value ||
+                                  bool(__is_final(typename Tsm::sm))> {};
 } // namespace aux
 
 namespace back {

test/ft/dependencies.cpp

@@ -477,3 +477,49 @@ test const_pointer_dep_lvalue_not_null = [] {
   sm.process_event(e1{});
   expect(sm.is(sml::X));
 };
+
+// Issue #483: dependencies (and SM classes themselves) declared `final` failed
+// to compile because `is_empty_base<T>` tried to inherit from T, and `sm_impl`
+// tried to subclass the SM class.
+// Fix (is_empty): add a partial specialisation guarded by __is_final(T) that
+//   reports false without ever instantiating is_empty_base<T>.
+// Fix (sm_impl): extend should_not_subclass_statemachine_class to also return
+//   true for final SM classes, routing them through the composition path.
+
+struct final_dep483 final {
+  int val = 99;
+};
+
+test final_dependency_type_compiles = [] {
+  // A dependency type declared `final` must be usable as an SM dependency.
+  struct c483dep {
+    auto operator()() noexcept {
+      using namespace sml;
+      auto action = [](final_dep483& d) { expect(99 == d.val); };
+      // clang-format off
+      return make_transition_table(*idle + event<e1> / action = X);
+      // clang-format on
+    }
+  };
+
+  final_dep483 dep;
+  sml::sm<c483dep> sm{dep};
+  sm.process_event(e1{});
+  expect(sm.is(sml::X));
+};
+
+test final_sm_class_compiles = [] {
+  // An SM class declared `final` must work via the composition path.
+  struct final_sm483 final {
+    auto operator()() noexcept {
+      using namespace sml;
+      // clang-format off
+      return make_transition_table(*idle + event<e2> = X);
+      // clang-format on
+    }
+  };
+
+  sml::sm<final_sm483> sm;
+  sm.process_event(e2{});
+  expect(sm.is(sml::X));
+};