fix(focus-trap): warn if initial element is not focusable (#13960)

Currently if the consumer sets the `cdkFocusInitial` group on a non-focusable element, nothing will happen. These changes add a warning if the app is running in dev mode.

Relates to #13953.

src/cdk/a11y/focus-trap/focus-trap.spec.ts

@@ -16,6 +16,7 @@ describe('FocusTrap', () => {
         FocusTrapWithSvg,
         FocusTrapWithoutFocusableElements,
         FocusTrapWithAutoCapture,
+        FocusTrapUnfocusableTarget,
       ],
     });
 
@@ -138,6 +139,18 @@ describe('FocusTrap', () => {
       focusTrapInstance.focusLastTabbableElement();
       expect(document.activeElement!.id).toBe('last');
     });
+
+    it('should warn if the initial focus target is not focusable', () => {
+      const alternateFixture = TestBed.createComponent(FocusTrapUnfocusableTarget);
+      alternateFixture.detectChanges();
+      focusTrapInstance = fixture.componentInstance.focusTrapDirective.focusTrap;
+
+      spyOn(console, 'warn');
+      focusTrapInstance.focusInitialElement();
+
+      expect(console.warn).toHaveBeenCalled();
+    });
+
   });
 
   describe('special cases', () => {
@@ -235,6 +248,16 @@ class FocusTrapTargets {
   @ViewChild(CdkTrapFocus) focusTrapDirective: CdkTrapFocus;
 }
 
+@Component({
+  template: `
+    <div cdkTrapFocus>
+      <div cdkFocusInitial></div>
+    </div>
+    `
+})
+class FocusTrapUnfocusableTarget {
+  @ViewChild(CdkTrapFocus) focusTrapDirective: CdkTrapFocus;
+}
 
 @Component({
   template: `

src/cdk/a11y/focus-trap/focus-trap.ts

@@ -18,6 +18,7 @@ import {
   NgZone,
   OnDestroy,
   DoCheck,
+  isDevMode,
 } from '@angular/core';
 import {take} from 'rxjs/operators';
 import {InteractivityChecker} from '../interactivity-checker/interactivity-checker';
@@ -189,6 +190,12 @@ export class FocusTrap {
                     `will be removed in 8.0.0`, redirectToElement);
       }
 
+      // Warn the consumer if the element they've pointed to
+      // isn't focusable, when not in production mode.
+      if (isDevMode() && !this._checker.isFocusable(redirectToElement)) {
+        console.warn(`Element matching '[cdkFocusInitial]' is not focusable.`, redirectToElement);
+      }
+
       redirectToElement.focus();
       return true;
     }