If the fill tool has nothing to fill (ie output=input) this is often an edge case and may indicate to the user that the workflow is incorrect. Please have the Fill tool generate a warning message when no fills are completed.
I could see this warning message being useful in an iterative workflow, for example, if a max fill depth is specified you may want to know whether that threshhold setting resulted in no fills without having to do a bunch of cell by cell comparisons after the fact.
