Thank you for the clarification! It was unclear to me at first if maybe it was some kind of infinite recursion problem, but I see now what you mean. As tye has already explained, the answer is clear: a comparator like that means the result of the sort is not well-defined.

Although I think in this case a solution other than sort is more appropriate, just to play out the idea in theory: If you wanted to sort by changing criteria, including criteria based on the position of each item in the list, then something resembling an iterative approach might be possible: 1. calculate and cache the sort criteria, 2. run the sort, 3. check if the ordering is appropriate and if not go back to step 1. That way, you're not violating the requirements of sort (the criteria remain stable during the entire run of sort), although you do have the problem that the algorithm might not converge overall.

Regards,
-- Hauke D