Would a change in priorities be considered breaking?

Also,
Do you envision different solvers having different priorities?
In this case the should have their own copy_to sorting with their priorities, and this default copy_to would be fine.

I think so. #2366 is really asking for a first-class priority order of attributes.

Perhaps Base.isless should be instead some official MOI.CopyToPriorityFactor or something.

But open questions are whether it should depend on the dest::MOI.ModelLike, or whether there is some solver-independent ordering.

The attributes really form a DAG, and we need a topological sort of them... It'd be nice to say that ObjectiveFunction depends on ObjectiveSense, and then have the order fall out of that.

We could maybe also have SortedListOfModelAttributes that could be used for solvers so that they don't need to worry about the sort. MOI.Utilities.Model could also implement a getter for this attribute quite easily.
Maybe we can also require solvers to always return it sorted but maybe an explicit sort here is safer.

SortedListOfModelAttributes is reasonable. But it means that it works only for attributes defined in MOI proper. Everything else would come at the end in an arbitrary order.

#2371 is a much cleaner approach.

Closing in favor of #2371