[BUGFIX release] Ensure Ember.isArray does not trigger proxy assertion.

packages/ember-runtime/lib/utils.js

@@ -1,3 +1,6 @@
+import { DEBUG } from '@glimmer/env';
+import { PROXY_CONTENT } from 'ember-metal';
+import { HAS_NATIVE_PROXY } from 'ember-utils';
 import EmberArray from './mixins/array';
 import EmberObject from './system/object';
 
@@ -48,7 +51,15 @@ const { toString } = Object.prototype;
   @return {Boolean} true if the passed object is an array or Array-like
   @public
 */
-export function isArray(obj) {
+export function isArray(_obj) {
+  let obj = _obj;
+  if (DEBUG && HAS_NATIVE_PROXY && typeof _obj === 'object' && _obj !== null) {
+    let possibleProxyContent = _obj[PROXY_CONTENT];
+    if (possibleProxyContent !== undefined) {
+      obj = possibleProxyContent;
+    }
+  }
+
   if (!obj || obj.setInterval) {
     return false;
   }

packages/ember-runtime/tests/core/is_array_test.js

@@ -1,6 +1,7 @@
 import { isArray } from '../../lib/utils';
 import { A as emberA } from '../../lib/mixins/array';
 import ArrayProxy from '../../lib/system/array_proxy';
+import EmberObject from '../../lib/system/object';
 import { window } from 'ember-browser-environment';
 import { moduleFor, AbstractTestCase } from 'internal-test-helpers';
 
@@ -32,6 +33,19 @@ moduleFor(
       assert.equal(isArray(arrayProxy), true, '[]');
     }
 
+    '@test Ember.isArray does not trigger proxy assertion when probing for length GH#16495'(
+      assert
+    ) {
+      let instance = EmberObject.extend({
+        // intentionally returning non-null / non-undefined
+        unknownProperty() {
+          return false;
+        },
+      }).create();
+
+      assert.equal(isArray(instance), false);
+    }
+
     ['@test Ember.isArray(fileList)'](assert) {
       if (window && typeof window.FileList === 'function') {
         let fileListElement = document.createElement('input');