fix(cdk/stepper): reset submitted state when resetting stepper

`CdkStepper` has a `reset` method that reset all the controls to their initial values, but that won't necessarily put the form into its initial state, because form controls also show errors on submit by default and `AbstractControl.reset` won't reset the submitted state.

These changes add a call to reset all child forms to their unsubmitted state.

Fixes #29781.

src/cdk/stepper/stepper.ts

@@ -33,7 +33,12 @@ import {
   numberAttribute,
   inject,
 } from '@angular/core';
-import {type AbstractControl} from '@angular/forms';
+import {
+  ControlContainer,
+  type AbstractControl,
+  type NgForm,
+  type FormGroupDirective,
+} from '@angular/forms';
 import {_getFocusedElementPierceShadowDom} from '@angular/cdk/platform';
 import {Observable, of as observableOf, Subject} from 'rxjs';
 import {startWith, takeUntil} from 'rxjs/operators';
@@ -101,7 +106,7 @@ export interface StepperOptions {
 @Component({
   selector: 'cdk-step',
   exportAs: 'cdkStep',
-  template: '<ng-template><ng-content></ng-content></ng-template>',
+  template: '<ng-template><ng-content/></ng-template>',
   encapsulation: ViewEncapsulation.None,
   changeDetection: ChangeDetectionStrategy.OnPush,
   standalone: true,
@@ -114,6 +119,19 @@ export class CdkStep implements OnChanges {
   /** Template for step label if it exists. */
   @ContentChild(CdkStepLabel) stepLabel: CdkStepLabel;
 
+  /** Forms that have been projected into the step. */
+  @ContentChildren(
+    // Note: we look for `ControlContainer` here, because both `NgForm` and `FormGroupDirective`
+    // provides themselves as such, but we don't want to have a concrete reference to both of
+    // the directives. The type is marked as `Partial` in case we run into a class that provides
+    // itself as `ControlContainer` but doesn't have the same interface as the directives.
+    ControlContainer,
+    {
+      descendants: true,
+    },
+  )
+  protected _childForms: QueryList<Partial<NgForm | FormGroupDirective>> | undefined;
+
   /** Template for step content. */
   @ViewChild(TemplateRef, {static: true}) content: TemplateRef<any>;
 
@@ -205,6 +223,10 @@ export class CdkStep implements OnChanges {
     }
 
     if (this.stepControl) {
+      // Reset the forms since the default error state matchers will show errors on submit and we
+      // want the form to be back to its initial state (see #29781). Submitted state is on the
+      // individual directives, rather than the control, so we need to reset them ourselves.
+      this._childForms?.forEach(form => form.resetForm?.());
       this.stepControl.reset();
     }
   }

tools/public_api_guard/cdk/stepper.md

@@ -10,9 +10,11 @@ import { AfterViewInit } from '@angular/core';
 import { ElementRef } from '@angular/core';
 import { EventEmitter } from '@angular/core';
 import { FocusableOption } from '@angular/cdk/a11y';
+import { FormGroupDirective } from '@angular/forms';
 import * as i0 from '@angular/core';
 import * as i1 from '@angular/cdk/bidi';
 import { InjectionToken } from '@angular/core';
+import { NgForm } from '@angular/forms';
 import { OnChanges } from '@angular/core';
 import { OnDestroy } from '@angular/core';
 import { QueryList } from '@angular/core';
@@ -24,6 +26,7 @@ export class CdkStep implements OnChanges {
     constructor(...args: unknown[]);
     ariaLabel: string;
     ariaLabelledby: string;
+    protected _childForms: QueryList<Partial<NgForm | FormGroupDirective>> | undefined;
     get completed(): boolean;
     set completed(value: boolean);
     // (undocumented)
@@ -60,7 +63,7 @@ export class CdkStep implements OnChanges {
     // (undocumented)
     _stepper: CdkStepper;
     // (undocumented)
-    static ɵcmp: i0.ɵɵComponentDeclaration<CdkStep, "cdk-step", ["cdkStep"], { "stepControl": { "alias": "stepControl"; "required": false; }; "label": { "alias": "label"; "required": false; }; "errorMessage": { "alias": "errorMessage"; "required": false; }; "ariaLabel": { "alias": "aria-label"; "required": false; }; "ariaLabelledby": { "alias": "aria-labelledby"; "required": false; }; "state": { "alias": "state"; "required": false; }; "editable": { "alias": "editable"; "required": false; }; "optional": { "alias": "optional"; "required": false; }; "completed": { "alias": "completed"; "required": false; }; "hasError": { "alias": "hasError"; "required": false; }; }, { "interactedStream": "interacted"; }, ["stepLabel"], ["*"], true, never>;
+    static ɵcmp: i0.ɵɵComponentDeclaration<CdkStep, "cdk-step", ["cdkStep"], { "stepControl": { "alias": "stepControl"; "required": false; }; "label": { "alias": "label"; "required": false; }; "errorMessage": { "alias": "errorMessage"; "required": false; }; "ariaLabel": { "alias": "aria-label"; "required": false; }; "ariaLabelledby": { "alias": "aria-labelledby"; "required": false; }; "state": { "alias": "state"; "required": false; }; "editable": { "alias": "editable"; "required": false; }; "optional": { "alias": "optional"; "required": false; }; "completed": { "alias": "completed"; "required": false; }; "hasError": { "alias": "hasError"; "required": false; }; }, { "interactedStream": "interacted"; }, ["stepLabel", "_childForms"], ["*"], true, never>;
     // (undocumented)
     static ɵfac: i0.ɵɵFactoryDeclaration<CdkStep, never>;
 }