Comment 1 for bug 1709396

code that's causing this ambiguity:

            static int initialize(void* dest, param_T operand)
            {
                typedef typename boost::detail::make_reference_content<
                      recursive_enabled_T
                    >::type internal_T;

                new(dest) internal_T(operand);
                return BOOST_MPL_AUX_VALUE_WKND(index)::value; // which
            }

#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
            static int initialize(void* dest, param2_T operand)
            {
                // This assert must newer trigger, because all the reference contents are
                // handled by the initilize(void* dest, param_T operand) function above
                BOOST_ASSERT(!is_reference_content_t::value);

                typedef typename boost::mpl::if_<is_reference_content_t, param2_T, recursive_enabled_T>::type value_T;
                new(dest) value_T( boost::detail::variant::move(operand) );
                return BOOST_MPL_AUX_VALUE_WKND(index)::value; // which
            }
#endif

gcc definition for this flag:

// C++0x features in 4.3.n and later
//
#if (BOOST_GCC_VERSION >= 40300) && defined(BOOST_GCC_CXX11)
// C++0x features are only enabled when -std=c++0x or -std=gnu++0x are
// passed on the command line, which in turn defines
// __GXX_EXPERIMENTAL_CXX0X__.
# define BOOST_HAS_DECLTYPE
# define BOOST_HAS_RVALUE_REFS
# define BOOST_HAS_STATIC_ASSERT
# define BOOST_HAS_VARIADIC_TMPL
#else
# define BOOST_NO_CXX11_DECLTYPE
# define BOOST_NO_CXX11_FUNCTION_TEMPLATE_DEFAULT_ARGS
# define BOOST_NO_CXX11_RVALUE_REFERENCES
# define BOOST_NO_CXX11_STATIC_ASSERT
#endif

and

#if defined(__GXX_EXPERIMENTAL_CXX0X__) || (__cplusplus >= 201103L)
# define BOOST_GCC_CXX11
#endif