Reordering constraints changing performance is basically impossible to avoid for constraint systems, because we are very bad at predicting which constraints sho

Reordering constraints changing performance is basically impossible to avoid for constraint systems, because we are very bad at predicting which constraints should be considered first -- well, you can always apply some normalisation pass which at least hides the problem from users. Reording causing changes in feasibility of a problem is a MAJOR bug, and I hoped they reported it. The constraint system I work on (Minion and Essence', a competitor to MiniZinc), has (AFAIK) never had such a bug in any release (they come up occasionally during development, but we have serious tests specially designed to detect that kind of problem).