fix(select): pointing to non-existent element via aria-labelledby (#12411)

Fixes `mat-select` referring to an element that doesn't exist, through `aria-labelledby`, if the select is inside a form field that doesn't have a label.

Fixes #12405.

src/lib/select/select.spec.ts

@@ -126,6 +126,7 @@ describe('MatSelect', () => {
         SelectWithGroups,
         SelectWithGroupsAndNgContainer,
         SelectWithFormFieldLabel,
+        SelectWithChangeEvent,
       ]);
     }));
 
@@ -255,6 +256,16 @@ describe('MatSelect', () => {
           expect(select.getAttribute('aria-labelledby')).toBeFalsy();
         });
 
+        it('should not set `aria-labelledby` if there is no form field label', () => {
+          fixture.destroy();
+
+          const labelFixture = TestBed.createComponent(SelectWithChangeEvent);
+          labelFixture.detectChanges();
+          select = labelFixture.debugElement.query(By.css('mat-select')).nativeElement;
+
+          expect(select.getAttribute('aria-labelledby')).toBeFalsy();
+        });
+
         it('should select options via the UP/DOWN arrow keys on a closed select', fakeAsync(() => {
           const formControl = fixture.componentInstance.control;
           const options = fixture.componentInstance.options.toArray();

src/lib/select/select.ts

@@ -1065,7 +1065,8 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
 
     // Note: we use `_getAriaLabel` here, because we want to check whether there's a
     // computed label. `this.ariaLabel` is only the user-specified label.
-    if (!this._parentFormField || this._getAriaLabel()) {
+    if (!this._parentFormField || !this._parentFormField._hasFloatingLabel() ||
+      this._getAriaLabel()) {
       return null;
     }