BUG: typeof内的表达式报告未定义

// example.wo
import woo.std;

func all<T>(iterable: T)
{
    let iterator = iterable->iter;

    alias elem_t = typeof(iterator->next->val);
    //                    ^^^^^^^^ Here!
    let elems = []mut: vec<elem_t>;

    while (true)
    {
        match(iterator->next)
        {
        value(e)? elems->add(e);
        none? break;
        }
    }
    return elems->unsafe::cast:<array<elem_t>>;
}
std::println(all([1,2,3,4,5,6,7,8,9]));