I'm getting a complaint of an ambiguity when I use BOOST_ENUM_VALUES with std::size_t as the type. eg: BOOST_ENM_VALUES(MyTypeName, std::size_t, (fred)(0) (jane)(1) ) The error is: E2015 Ambiguity between 'boost::optional<unsigned int>::optional(int boost::detail::none_helper::* const &)' and 'boost::optional<unsigned int>::optional(const unsigned int &)' Which is in the static method 'values' for the case where the index is zero. The line is: return optional_value(0); This does not happen if I use 'int', but does also happen with 'unsigned int'. A guess is that the compiler thinks that the zero might be null pointer? A possible solution is to do a cast: return optional_value(static_cast<value_type>(0)); This only happens for the zero value, other values are fine.