I don't believe it's guaranteed to work correctly on any system, because in this code:

    is-member(i):
    return sparse[i] < n && dense[sparse[i]] == i

there's no requirement for the two sparse[i] to take the same value (since that expression evaluates to an indeterminate vaule). So the second occurrence of sparse[i] can take a value >=n, or indeed greater than the length of the whole array, at which point reading it is definitely UB.