TreeUpdateResult is currently very small, up that doesn't mean it always will be. Currently I see no value in passing it by value. Passing by const reference should be the default for structs and I don't see a reason to make an exception here.

Also, it is stored by value throughout the update code. I'm not sure if this could be a confusion, since as far as I understand, the map that owns the original doesn't seem to change anymore.

Hm, fair enough, don't mind much either way.