[Bug] {{each}} item binding with square brackets doesn't update, when right side is an aliased attribute

[ericyhwang]

The title is long and complicated. I have a repro of the issue using Derby's component tests:
ee2e1b0

Also putting the example here:

<Body:>
  <view is="greeting-list" greetingIds="{{_page.greetingIds}}"/>

<greeting-list:>
  {{each @greetingIds as #id, #i}}
    <div>{{_page.greetings[#id]}}</div>
  {{/each}}

Doing a model.set('_page.greetingIds.0', 'foo'), I'd expect the binding to update, but it doesn't.

The binding does update properly under these other scenarios:

1. Setting the entire array: model.set('_page.greetingIds', ['foo'])
2. Changing the template to use the array index: {{ _page.greetings[@greetingIds[#i]] }}
3. Changing the template to not use an attribute: {{each _page.greetingIds as #id, #i}}

[ericyhwang]

Based on the fact that {{each _page.greetingIds}} does work, I've debugged it down to a difference between PathExpression#dependencies (works fine) and AttributePathExpression#dependencies (doesn't work).

In the screenshot, the left side is the working PathExpression, and the right side is the not-working AttributePathExpression.

Specifically, PathExpression's call to appendDependency results in a correct dep list, but AttributePathExpression's call to swapLastDependency does not.

---

As for a fix - Just to see what happens, since I suspected the early-return to be an issue, I tried changing the initial early-return check in swapLastDependency:

https://github.com/derbyjs/derby-templates/blob/6cf8f1ba344c9aec59e577945efe4d17416fcd10/lib/expressions.js#L748

Like so:

  function swapLastDependency(dependencies, expression, context) {
-   if (!expression.segments.length) {
+   if (!expression.segments.length && !expression.meta) {
      return dependencies;
    }

That did fix the issue, and all the other tests pass... but I have no idea what the intent behind swapLastDependency is, nor do I know what that early-return is intended to do. My "see what happens" change is almost certainly not the correct fix.

@nateps can you provide some context / advise?

[JerryI]

It was an old bug which made me kill myself at the previous job. Thank you a ton, @ericyhwang!