fix(select): remove aria-owns when options aren't in the DOM

Removes the select's `aria-owns` attribute when the options aren't in the DOM, in order to avoid pointing non-existing elements.

Fixes angular#7023.

src/lib/select/select.spec.ts

@@ -1657,6 +1657,22 @@ describe('MatSelect', () => {
             .toContain(options[1].id, `Expected aria-owns to contain IDs of its child options.`);
       }));
 
+      it('should remove aria-owns when the options are not visible', fakeAsync(() => {
+        const select = fixture.debugElement.query(By.css('mat-select'));
+
+        expect(select.nativeElement.hasAttribute('aria-owns'))
+            .toBe(true, 'Expected select to have aria-owns while open.');
+
+        const backdrop =
+            overlayContainerElement.querySelector('.cdk-overlay-backdrop') as HTMLElement;
+        backdrop.click();
+        fixture.detectChanges();
+        flush();
+
+        expect(select.nativeElement.hasAttribute('aria-owns'))
+            .toBe(false, 'Expected select not to have aria-owns when closed.');
+      }));
+
       it('should set the option id properly', fakeAsync(() => {
         let firstOptionID = options[0].id;
 

src/lib/select/select.ts

@@ -188,7 +188,7 @@ export class MatSelectTrigger {}
     '[attr.aria-required]': 'required.toString()',
     '[attr.aria-disabled]': 'disabled.toString()',
     '[attr.aria-invalid]': 'errorState',
-    '[attr.aria-owns]': '_optionIds',
+    '[attr.aria-owns]': 'panelOpen ? _optionIds : null',
     '[attr.aria-multiselectable]': 'multiple',
     '[attr.aria-describedby]': '_ariaDescribedby || null',
     '[attr.aria-activedescendant]': '_getAriaActiveDescendant()',