Example project 'nullable_enum_binding' cannot compile

[juandent]

This code is in error:

    /**
     *  This is where sqlite_orm lib understands that your type is nullable - by
     *  specializing type_is_nullable<T> and deriving from std::true_type.
     */
    template<>
    struct type_is_nullable<Gender> : public std::true_type {

        //  this function must return whether value null or not (false is null). Don't forget to implement it
        bool operator()(const Gender& g) const {
            return g != Gender::None;
        }
    };

because type_is_nullable is defined like so:

    template<class T>
    using type_is_nullable = polyfill::disjunction<
#ifdef SQLITE_ORM_OPTIONAL_SUPPORTED
        polyfill::is_specialization_of<T, std::optional>,
#endif
        polyfill::is_specialization_of<T, std::unique_ptr>,
        polyfill::is_specialization_of<T, std::shared_ptr>>;

The VS2022 gives this error:

alias cannot be redeclared as class

How can we define an enum as nullable? By using std::optional<Gender> where Gender is an enum?