fix null deref when pointer dependency passed as lvalue to sm ctor (#485)

[PavelGuzenfeld]

Problem

When a T* dependency is passed as an lvalue to sm<M>{ptr}, C++ forwarding reference deduction binds TDeps as T*& (reference-to-pointer). The pool init constructor then builds an input pool of type pool<T*&>.

The stripping logic in the pool init constructor:

try_get<remove_const_t<remove_reference_t<remove_pointer_t<Ts>>>>(&p)

strips T* → T, then calls try_get<T> on pool<T*&>. The existing try_get overloads only match pool_type<T*> and pool_type<const T*> — there is no overload for pool_type<T*&>, so the call falls through to the variadic catch-all and returns missing_ctor_parameter<T>{}. That silently converts to a null pointer. Every action or guard that takes a pointer dependency then receives nullptr and dereferences it → crash / assertion failure.

Reproduction

struct Dep { int val = 42; };
struct M {
  auto operator()() noexcept {
    using namespace sml;
    // action takes const Dep* — was receiving nullptr
    auto action = [](const Dep* d) { assert(d != nullptr); };
    return make_transition_table(*"s1"_s + event<e1>{} / action = X);
  }
};
Dep dep;
Dep* ptr = &dep;          // lvalue pointer
sml::sm<M> sm{ptr};       // TDeps deduced as Dep*& — bug triggers here
sm.process_event(e1{});   // assertion fires

The same bug affects Dep* (non-const) action parameters.

Fix

Add two try_get overloads that match reference-to-pointer pool slots:

template <class T>
constexpr T *try_get(const pool_type<T *&> *object) {
  return object->value;
}
template <class T>
constexpr const T *try_get(const pool_type<const T *&> *object) {
  return object->value;
}

These extract the stored pointer from the reference-wrapping slot. The T& (reference dependency) case already worked correctly and is unaffected.

Tests

Regression tests added in test/ft/dependencies.cpp:

• pointer_dep_lvalue_not_null — action takes Dep*, SM constructed with Dep* lvalue
• const_pointer_dep_lvalue_not_null — action takes const Dep*, SM constructed with Dep* lvalue

Existing test suite passes without modification.

Fixes #485.