Method type operator=

extern/boost/boost/optional/optional.hpp:1055–1060

    template<class Expr>
    BOOST_DEDUCED_TYPENAME boost::enable_if<optional_detail::is_optional_val_assign_candidate<T, Expr>, optional&>::type
    operator= ( Expr&& expr )
      {
        this->assign_expr(boost::forward<Expr>(expr),boost::addressof(expr));
        return *this ;
      }

#else
    template<class Expr>