So this is basically a shortcut for(val.id || val.subItem.id)? But in which cases exactly do you need this?
From your changes to the docs, that's not entirely clear, so I would like a better description of this feature.
It's also not enough to change the existing tests, there should be new tests that make explicit what this feature does.

So this is basically a shortcut for (val.id || val.subItem.id)?

In the simple case, yes.

But in which cases exactly do you need this?

Here's a contrived example (note the id's don't match up).

$scope.items=[{id:0,label:'aLabel',subItem:{name:'aSubItem',id:1}},{id:1,label:'bLabel',subItem:{name:'bSubItem',id:0}}];$scope.selected=$scope.items[0].subItem;

If you use(val.id || val.subItem.id) here, the wrong object will be initially selected.

demo:https://plnkr.co/edit/8MQXJpRqEdtwF1hcCM3w?p=preview

edit: In my actual use case thetrack by expression was(item.valueOf() || item.subItem.valueOf()) and that wont work because all objects have that method.

@Narretz I noticed you put this in the Ice Box milestone and I don't want to invest more time into this if it's not going anywhere. I think that I presented a valid use case in my last comment, but if that's not the case, let me know so I don't waste my time.

@Narretz I've add a bit more to the documentation, but I'm not sure about the tests. Here's what it says above the tests I changed.

  /**   * This behavior is broken and should probably be cleaned up later as track by and select as   * aren't compatible.   */

This is what the$value variable aims to fix. So it doesn't make sense to still have these tests using the hacky workaround. If I was going to add tests, they would be identical except for how thetrack by expression is written.