[BUGFIX canary] Ensure uninitialized tracked properties "work".

packages/@ember/-internals/glimmer/tests/integration/components/tracked-test.js

@@ -9,6 +9,32 @@ if (EMBER_METAL_TRACKED_PROPERTIES) {
   moduleFor(
     'Component Tracked Properties',
     class extends RenderingTestCase {
+      '@test tracked properties that are uninitialized do not throw an error'() {
+        let CountComponent = Component.extend({
+          count: tracked(),
+
+          increment() {
+            if (!this.count) {
+              this.count = 0;
+            }
+            this.count++;
+          },
+        });
+
+        this.registerComponent('counter', {
+          ComponentClass: CountComponent,
+          template: '<button {{action this.increment}}>{{this.count}}</button>',
+        });
+
+        this.render('<Counter />');
+
+        this.assertText('');
+
+        runTask(() => this.$('button').click());
+
+        this.assertText('1');
+      }
+
       '@test tracked properties rerender when updated'() {
         let CountComponent = Component.extend({
           count: tracked({ value: 0 }),

packages/@ember/-internals/metal/lib/tracked.ts

@@ -205,7 +205,7 @@ function descriptorForField([_target, key, desc]: [
 
       // If the field has never been initialized, we should initialize it
       if (!(secretKey in this)) {
-        this[secretKey] = initializer !== undefined ? initializer.call(this) : undefined;
+        this[secretKey] = typeof initializer === 'function' ? initializer.call(this) : undefined;
       }
 
       return this[secretKey];

packages/@ember/-internals/metal/tests/tracked/classic_classes_test.js

@@ -156,6 +156,16 @@ if (EMBER_METAL_TRACKED_PROPERTIES) {
             new Tracked();
           }, "You attempted to set a default value for first with the @tracked({ value: 'default' }) syntax. You can only use this syntax with classic classes. For native classes, you can use class initializers: @tracked field = 'default';");
         }
+
+        [`@test errors if options are passed to native decorator (GH#17764)`](assert) {
+          class Tracked {
+            @tracked value;
+          }
+
+          let obj = new Tracked();
+
+          assert.strictEqual(obj.value, undefined, 'uninitilized value defaults to undefined');
+        }
       }
     );
   } else {