add constexpr specifier

Author: uyha

include/boost/sml.hpp

@@ -2443,7 +2443,7 @@ struct transition<state<S2>, transition_eg<front::event<E>, G>>
  constexpr transition(const state<S2> &, const transition_eg<front::event<E>, G> &t)
  : transition<state<internal>, state<S2>, front::event<E>, G, none>{t.g, none{}} {}
  template <class T>
- auto operator=(const T &) const {
+ constexpr auto operator=(const T &) const {
  return transition<T, state<S2>, front::event<E>, G, none>{g, none{}};
  }
 };
@@ -2460,7 +2460,7 @@ struct transition<state<S2>, transition_ea<front::event<E>, A>>
  constexpr transition(const state<S2> &, const transition_ea<front::event<E>, A> &t)
  : transition<state<internal>, state<S2>, front::event<E>, always, A>{always{}, t.a} {}
  template <class T>
- auto operator=(const T &) const {
+ constexpr auto operator=(const T &) const {
  return transition<T, state<S2>, front::event<E>, always, A>{always{}, a};
  }
 };