This constraint implements an arc-consistent version of X #= Y + C (i.e. any "holes" in the domain of X are propagated to Y and vice-versa).
A range error is thrown if X and Y do not have "reasonable" bounds (roughly +/- 2^30 on a 32-bit machine, roughly +/- 2^52 on a 64-bit machine: note that even much smaller bounds than this can lead to available memory being exhausted very rapidly, if not immediately).