fix(select): complete state change event (#8777)

* fix(select): complete state change event

* chore: add event test

src/lib/select/select.spec.ts

@@ -1778,6 +1778,25 @@ describe('MatSelect', () => {
     }));
   });
 
+  describe('change events', () => {
+    beforeEach(async(() => configureMatSelectTestingModule([SelectWithPlainTabindex])));
+
+    it('should complete the stateChanges stream on destroy', () => {
+      const fixture = TestBed.createComponent(SelectWithPlainTabindex);
+      fixture.detectChanges();
+
+      const debugElement = fixture.debugElement.query(By.directive(MatSelect));
+      const select = debugElement.componentInstance;
+
+      const spy = jasmine.createSpy('stateChanges complete');
+      const subscription = select.stateChanges.subscribe(undefined, undefined, spy);
+
+      fixture.destroy();
+      expect(spy).toHaveBeenCalled();
+      subscription.unsubscribe();
+    });
+  });
+
   describe('when initially hidden', () => {
     beforeEach(async(() => configureMatSelectTestingModule([BasicSelectInitiallyHidden])));
 

src/lib/select/select.ts

@@ -509,6 +509,7 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
   ngOnDestroy() {
     this._destroy.next();
     this._destroy.complete();
+    this.stateChanges.complete();
   }
 
   /** Toggles the overlay panel open or closed. */