Failing test for #14672

PROPERTY_DID_CHANGE is the mechanism by which
Ember's Glimmer refs are dirtied.  Currently if
something watches then unwatches an alias, the DK
that is installed by the alias is removed.

packages/ember-metal/tests/watching/watch_test.js

@@ -3,14 +3,19 @@ import { meta } from '../../meta';
 import { set } from '../../property_set';
 import get from '../../property_get';
 import { computed } from '../../computed';
+import alias from '../../alias';
 import { defineProperty } from '../../properties';
 import { testBoth } from 'internal-test-helpers';
 import { addListener } from '../../events';
 import {
   watch,
   unwatch,
-  destroy
+  destroy,
+  isWatching
 } from '../../watching';
+import {
+  PROPERTY_DID_CHANGE
+} from '../../property_events';
 
 let willCount, didCount, willKeys, didKeys, originalLookup;
 
@@ -253,3 +258,60 @@ testBoth('watch + Ember.set + no-descriptor', function(get, set) {
   equal(child.b, 1, 'child.b (after watch)');
   equal(get(child, 'b'), 1, 'Ember.get(child, "b") (after watch)');
 });
+
+testBoth('PROPERTY_DID_CHANGE + alias + cp', function(get, set) {
+  let counts = {};
+  let obj = {
+    child: {},
+    [PROPERTY_DID_CHANGE](keyName) {
+      counts[keyName] = (counts[keyName] || 0) + 1;
+    }
+  };
+
+  defineProperty(obj, 'cost', alias('child.cost'));
+  defineProperty(obj, 'tax', alias('child.tax'));
+
+  defineProperty(obj, 'total', computed('cost', 'tax', {
+    get() {
+      return get(this, 'cost') + get(this, 'tax');
+    }
+  }));
+
+
+  ok(!isWatching(obj, 'child.cost'), 'precond alias target `child.cost` is not watched');
+  equal(get(obj, 'cost'), undefined);
+  // this is how PROPERTY_DID_CHANGE will get notified
+  ok(isWatching(obj, 'child.cost'), 'alias target `child.cost` is watched after consumption');
+
+  ok(!isWatching(obj, 'child.tax'), 'precond alias target `child.cost` is not watched');
+  equal(get(obj, 'tax'), undefined);
+  // this is how PROPERTY_DID_CHANGE will get notified
+  ok(isWatching(obj, 'child.tax'), 'alias target `child.cost` is watched after consumption');
+
+  // increments the watching count on the alias itself to 1
+  ok(isNaN(get(obj, 'total')), 'total is initialized');
+
+  // decrements the watching count on the alias itself to 0
+  set(obj, 'child', {
+    cost: 399.00,
+    tax: 32.93
+  });
+
+  // this should have called PROPERTY_DID_CHANGE for all of them
+  equal(counts['cost'], 1,  'PROPERTY_DID_CHANGE called with cost');
+  equal(counts['tax'], 1,   'PROPERTY_DID_CHANGE called with tax');
+  equal(counts['total'], 1, 'PROPERTY_DID_CHANGE called with total');
+
+  // we should still have a dependency installed
+  ok(isWatching(obj, 'child.cost'), 'watching child.cost');
+  ok(isWatching(obj, 'child.tax'), 'watching child.tax');
+
+  set(obj, 'child', {
+    cost: 100.00,
+    tax: 10.00
+  });
+
+  equal(counts['cost'], 2,  'PROPERTY_DID_CHANGE called with cost');
+  equal(counts['tax'], 2,   'PROPERTY_DID_CHANGE called with tax');
+  equal(counts['total'], 1, 'PROPERTY_DID_CHANGE called with total');
+});