fix(cdk/stepper): error if out-of-bounds index is assigned before initialization (#20766)

Fixes an error if an out-of-bounds index is assigned before the steps are
available and the user tries to navigate. Usually we have an assertion that
prevents invalid indexes from being assigned, but it doesn't run before
initialization, because we don't know how many steps there will be yet.

These changes add a check that will revert the index back to 0 if the index
is invalid on initialization.

Fixes #20735.

(cherry picked from commit d3a2718)

Author: crisbeto

src/cdk/stepper/stepper.ts

@@ -293,12 +293,11 @@ export class CdkStepper implements AfterContentInit, AfterViewInit, OnDestroy {
 
     if (this.steps && this._steps) {
       // Ensure that the index can't be out of bounds.
-      if ((newIndex < 0 || newIndex > this.steps.length - 1) &&
-        (typeof ngDevMode === 'undefined' || ngDevMode)) {
+      if (!this._isValidIndex(index) && (typeof ngDevMode === 'undefined' || ngDevMode)) {
         throw Error('cdkStepper: Cannot assign out-of-bounds value to `selectedIndex`.');
       }
 
-      if (this._selectedIndex != newIndex && !this._anyControlsInvalidOrPending(newIndex) &&
+      if (this._selectedIndex !== newIndex && !this._anyControlsInvalidOrPending(newIndex) &&
           (newIndex >= this._selectedIndex || this.steps.toArray()[newIndex].editable)) {
         this._updateSelectedItemIndex(index);
       }
@@ -365,6 +364,13 @@ export class CdkStepper implements AfterContentInit, AfterViewInit, OnDestroy {
         this._selectedIndex = Math.max(this._selectedIndex - 1, 0);
       }
     });
+
+    // The logic which asserts that the selected index is within bounds doesn't run before the
+    // steps are initialized, because we don't how many steps there are yet so we may have an
+    // invalid index on init. If that's the case, auto-correct to the default so we don't throw.
+    if (!this._isValidIndex(this._selectedIndex)) {
+      this._selectedIndex = 0;
+    }
   }
 
   ngOnDestroy() {
@@ -525,6 +531,11 @@ export class CdkStepper implements AfterContentInit, AfterViewInit, OnDestroy {
     return stepperElement === focusedElement || stepperElement.contains(focusedElement);
   }
 
+  /** Checks whether the passed-in index is a valid step index. */
+  private _isValidIndex(index: number): boolean {
+    return index > -1 && (!this.steps || index < this.steps.length);
+  }
+
   static ngAcceptInputType_editable: BooleanInput;
   static ngAcceptInputType_optional: BooleanInput;
   static ngAcceptInputType_completed: BooleanInput;

src/material/stepper/stepper.spec.ts

@@ -29,6 +29,7 @@ import {
   Provider,
   ViewChildren,
   QueryList,
+  ViewChild,
 } from '@angular/core';
 import {ComponentFixture, fakeAsync, flush, inject, TestBed} from '@angular/core/testing';
 import {
@@ -1270,6 +1271,24 @@ describe('MatStepper', () => {
     expect(steppers[0].steps.length).toBe(3);
     expect(steppers[1].steps.length).toBe(2);
   });
+
+  it('should not throw when trying to change steps after initializing to an out-of-bounds index',
+    () => {
+      const fixture = createComponent(StepperWithStaticOutOfBoundsIndex);
+      fixture.detectChanges();
+      const stepper = fixture.componentInstance.stepper;
+
+      expect(stepper.selectedIndex).toBe(0);
+      expect(stepper.selected).toBeTruthy();
+
+      expect(() => {
+        stepper.selectedIndex = 1;
+        fixture.detectChanges();
+      }).not.toThrow();
+
+      expect(stepper.selectedIndex).toBe(1);
+      expect(stepper.selected).toBeTruthy();
+    });
 });
 
 /** Asserts that keyboard interaction works correctly. */
@@ -1780,3 +1799,17 @@ class StepperWithNgIf {
 class NestedSteppers {
   @ViewChildren(MatStepper) steppers: QueryList<MatStepper>;
 }
+
+
+@Component({
+  template: `
+    <mat-vertical-stepper selectedIndex="1337">
+      <mat-step label="Step 1">Content 1</mat-step>
+      <mat-step label="Step 2">Content 2</mat-step>
+      <mat-step label="Step 3">Content 3</mat-step>
+    </mat-vertical-stepper>
+  `
+})
+class StepperWithStaticOutOfBoundsIndex {
+  @ViewChild(MatStepper) stepper: MatStepper;
+}