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

[PavelGuzenfeld]

Problem

Closes #483.

Types declared final cannot be used either as SM dependencies or as the SM class itself.
The compiler rejects the code because SML internally tries to inherit from the type in two places.

Root Causes & Fixes

1. is_empty<T> — is_empty_base<T> inherits from T

template <class T, class U>
struct is_empty_base : T { U _; };  // ← fails when T is final

Fix: add a partial specialisation guarded by __is_final(T) that inherits from false_type instead, so is_empty_base<T> is never instantiated for final types.

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 {};

2. sm_impl<Tsm> : Tsm::sm — sm_impl inherits from the SM class

struct sm_impl : aux::conditional_t<aux::should_not_subclass_statemachine_class<Tsm>::value,
                                     aux::none_type, typename Tsm::sm> {  // ← fails when sm is final

should_not_subclass_statemachine_class previously only returned true for empty SM classes (EBO) or when the dont_instantiate_statemachine_class policy was set.

Fix: extend the predicate to also return true when __is_final(Tsm::sm) is true. This routes final SM classes through the existing composition path (try_get_without_instantiating) which calls operator() on the SM object without requiring inheritance.

struct should_not_subclass_statemachine_class
    : integral_constant<bool, is_empty<typename Tsm::sm>::value ||
                              should_not_instantiate_statemachine_class<Tsm>::value ||
                              bool(__is_final(typename Tsm::sm))> {};

Notes

__is_final is a compiler builtin available on GCC, Clang, and MSVC — used here consistently with __is_base_of already present in the same header.

Tests

Two regression tests added to test/ft/dependencies.cpp:

• final_dependency_type_compiles — final dep type used in an action lambda
• final_sm_class_compiles — SM class itself declared final