[BUGFIX release] Fixes ArrayProxy content updates

ArrayProxy was not properly updating its own chain tags whenever its
content updated, which caused the regression.

Fixing this caused another issue, since we were updating tags with more
recent state and not properly dirtying those tags. Since those tags were
consumed eagerly, earlier on in the render cycle, there was no way to
dirty the tags when the update occured lazily. So, we had to add a
`CUSTOM_TAG_FOR` to trigger the update _when_ the tag was consumed.

packages/@ember/-internals/glimmer/tests/integration/syntax/each-test.js

@@ -1,6 +1,6 @@
 import { moduleFor, RenderingTestCase, applyMixins, strip, runTask } from 'internal-test-helpers';
 
-import { get, set, notifyPropertyChange, computed } from '@ember/-internals/metal';
+import { get, set, notifyPropertyChange, computed, on } from '@ember/-internals/metal';
 import { A as emberA, ArrayProxy, RSVP } from '@ember/-internals/runtime';
 import { HAS_NATIVE_SYMBOL } from '@ember/-internals/utils';
 
@@ -1108,6 +1108,22 @@ moduleFor(
   }
 );
 
+moduleFor(
+  'Syntax test: {{#each}} with array proxies, content is updated after init',
+  class extends EachTest {
+    createList(items) {
+      let wrapped = emberA(items);
+      let proxy = ArrayProxy.extend({
+        setup: on('init', function() {
+          this.set('content', emberA(wrapped));
+        }),
+      }).create();
+
+      return { list: proxy, delegate: wrapped };
+    }
+  }
+);
+
 moduleFor(
   'Syntax test: {{#each as}} undefined path',
   class extends RenderingTestCase {

packages/@ember/-internals/runtime/lib/system/array_proxy.js

@@ -12,13 +12,13 @@ import {
   replace,
   getChainTagsForKey,
   tagForProperty,
+  CUSTOM_TAG_FOR,
   arrayContentDidChange,
-  arrayContentWillChange,
 } from '@ember/-internals/metal';
 import EmberObject from './object';
 import { isArray, MutableArray } from '../mixins/array';
 import { assert } from '@ember/debug';
-import { combine, update, validate, value } from '@glimmer/validator';
+import { combine, dirty, update, validate, value, tagFor } from '@glimmer/validator';
 
 const ARRAY_OBSERVER_MAPPING = {
   willChange: '_arrangedContentArrayWillChange',
@@ -124,6 +124,16 @@ export default class ArrayProxy extends EmberObject {
     this._removeArrangedContentArrayObserver();
   }
 
+  [CUSTOM_TAG_FOR](key) {
+    if (key === '[]') {
+      // revalidate eagerly if we're being tracked, since we no longer will
+      // be able to later on due to backtracking re-render assertion
+      this._revalidate();
+    }
+
+    return tagFor(this, key);
+  }
+
   /**
     The content array. Must be an object that implements `Array` and/or
     `MutableArray.`
@@ -311,6 +321,15 @@ export default class ArrayProxy extends EmberObject {
 
       this._arrangedContentTag = combine(getChainTagsForKey(this, 'arrangedContent'));
       this._arrangedContentRevision = value(this._arrangedContentTag);
+
+      let arrTag = tagForProperty(this, '[]');
+      let lengthTag = tagForProperty(this, 'length');
+
+      update(arrTag, combine(getChainTagsForKey(this, 'arrangedContent.[]')));
+      update(lengthTag, combine(getChainTagsForKey(this, 'arrangedContent.length')));
+
+      dirty(arrTag);
+      dirty(lengthTag);
     }
   }
 }
@@ -328,10 +347,6 @@ ArrayProxy.reopen(MutableArray, {
 
   // Array proxies don't need to notify when they change since their `[]` tag is
   // already dependent on the `[]` tag of `arrangedContent`
-  arrayContentWillChange(startIdx, removeAmt, addAmt) {
-    return arrayContentWillChange(this, startIdx, removeAmt, addAmt, false);
-  },
-
   arrayContentDidChange(startIdx, removeAmt, addAmt) {
     return arrayContentDidChange(this, startIdx, removeAmt, addAmt, false);
   },