fix: guard taking const State& now sees live pool state (issue #530)

[PavelGuzenfeld]

Problem

When a state machine uses BOOST_SML_CREATE_DEFAULT_CONSTRUCTIBLE_DEPS and an action mutates a state object via State& while a subsequent guard reads it via const State&, the guard received a default-constructed copy instead of the live, mutated value.

Root cause: ignore::non_events produced two separate dep_list entries — State& and const State& — which caused two independent pool slots to be allocated and initialised from the same default-constructed object. Mutations via the mutable slot (State&) were invisible to the const-ref slot (const State&).

Minimal repro

#define BOOST_SML_CREATE_DEFAULT_CONSTRUCTIBLE_DEPS
#include <boost/sml.hpp>
struct State { int id{}; };
struct connect { int id{}; };

struct c {
  auto operator()() noexcept {
    using namespace sml;
    const auto set   = [](const connect& ev, State& s) { s.id = ev.id; };
    const auto check = [](const connect& ev, const State& s) {
      return s.id == 42; // FAILED: s.id was 0 (stale copy)
    };
    return make_transition_table(
      *state<struct idle> + event<connect> / set         = state<State>,
       state<State>       + event<connect>[check]        = X
    );
  }
};

sml::sm<c> sm;
sm.process_event(connect{42});  // sets State::id = 42
sm.process_event(connect{99});  // guard should see id==42, but saw 0

Fix (two parts)

1. ignore::non_events normalisation — map const U& → U& before inserting into the dep_list, so mutable and const references share a single pool slot:

template <class T>
struct non_events {
  using stripped_ = aux::remove_const_t<aux::remove_reference_t<T>>;
  using norm_ = aux::conditional_t<aux::is_same<T, const stripped_&>::value, stripped_&, T>;
  using type =
      aux::conditional_t<..., aux::type_list<>, aux::type_list<norm_>>;
};

2. New get_arg overload for const T& — looks up T& in the pool (not const T&) so both call-sites bind to the same live object:

template <class T, class TEvent, class Tsm, class TDeps,
          __BOOST_SML_REQUIRES(!aux::is_same<..., T>::value)>
constexpr const T& get_arg(const aux::type_wrapper<const T&>&,
                            const TEvent&, Tsm&, TDeps& deps) {
  return aux::get<T&>(deps);
}

Test

test const_ref_state_dep_sees_live_state added to test/ft/dependencies.cpp (compiled under BOOST_SML_CREATE_DEFAULT_CONSTRUCTIBLE_DEPS).

Closes #530

[jefftrull]

This has been a thorn in my side, thanks for repairing it!