fix(material/form-field): trigger CD when form gets reassigned

src/material/form-field/form-field.ts

@@ -33,7 +33,7 @@ import {
   contentChild,
   inject,
 } from '@angular/core';
-import {AbstractControlDirective} from '@angular/forms';
+import {AbstractControlDirective, ValidatorFn} from '@angular/forms';
 import {ThemePalette} from '@angular/material/core';
 import {_IdGenerator} from '@angular/cdk/a11y';
 import {Subject, Subscription, merge} from 'rxjs';
@@ -322,6 +322,7 @@ export class MatFormField
   private _explicitFormFieldControl: MatFormFieldControl<any>;
   private _needsOutlineLabelOffsetUpdate = false;
   private _previousControl: MatFormFieldControl<unknown> | null = null;
+  private _previousControlValidatorFn: ValidatorFn | null = null;
   private _stateChanges: Subscription | undefined;
   private _valueChanges: Subscription | undefined;
   private _describedByChanges: Subscription | undefined;
@@ -374,10 +375,30 @@ export class MatFormField
   ngAfterContentChecked() {
     this._assertFormFieldControl();
 
+    // if form field was being used with an input in first place and then replaced by other
+    // component such as select.
     if (this._control !== this._previousControl) {
       this._initializeControl(this._previousControl);
+
+      // keep a reference for last validator we had.
+      if (this._control.ngControl && this._control.ngControl.control) {
+        this._previousControlValidatorFn = this._control.ngControl.control.validator;
+      }
+
       this._previousControl = this._control;
     }
+
+    // make sure the the control has been initialized.
+    if (this._control.ngControl && this._control.ngControl.control) {
+      // get the validators for current control.
+      const validatorFn = this._control.ngControl.control.validator;
+
+      // if our current validatorFn isn't equal to it might be we are CD behind, marking the
+      // component will allow us to catch up.
+      if (validatorFn !== this._previousControlValidatorFn) {
+        this._changeDetectorRef.markForCheck();
+      }
+    }
   }
 
   ngOnDestroy() {

src/material/input/input.spec.ts

@@ -349,6 +349,48 @@ describe('MatMdcInput without forms', () => {
     expect(label.nativeElement.querySelector('.mat-mdc-form-field-required-marker')).toBeTruthy();
   }));
 
+  it('should show the required star when FormControl is reassigned', fakeAsync(() => {
+    const fixture = createComponent(MatInputWithRequiredAssignableFormControl);
+    fixture.detectChanges();
+
+    // should have star by default
+    let label = fixture.debugElement.query(By.css('label'))!;
+    expect(label.nativeElement.querySelector('.mat-mdc-form-field-required-marker')).toBeTruthy();
+
+    fixture.componentInstance.reassignFormControl();
+    fixture.changeDetectorRef.markForCheck();
+    fixture.detectChanges();
+
+    // should be removed as form was reassigned with no required validators
+    label = fixture.debugElement.query(By.css('label'))!;
+    expect(label.nativeElement.querySelector('.mat-mdc-form-field-required-marker')).toBeFalsy();
+  }));
+
+  it('should show the required star when required validator is toggled', fakeAsync(() => {
+    const fixture = createComponent(MatInputWithRequiredAssignableFormControl);
+    fixture.detectChanges();
+
+    // should have star by default
+    let label = fixture.debugElement.query(By.css('label'))!;
+    expect(label.nativeElement.querySelector('.mat-mdc-form-field-required-marker')).toBeTruthy();
+
+    fixture.componentInstance.removeRequiredValidtor();
+    fixture.changeDetectorRef.markForCheck();
+    fixture.detectChanges();
+
+    // should be removed as control validator was removed
+    label = fixture.debugElement.query(By.css('label'))!;
+    expect(label.nativeElement.querySelector('.mat-mdc-form-field-required-marker')).toBeFalsy();
+
+    fixture.componentInstance.addRequiredValidator();
+    fixture.changeDetectorRef.markForCheck();
+    fixture.detectChanges();
+
+    // should contain star as control validator was readded
+    label = fixture.debugElement.query(By.css('label'))!;
+    expect(label.nativeElement.querySelector('.mat-mdc-form-field-required-marker')).toBeTruthy();
+  }));
+
   it('should not hide the required star if input is disabled', () => {
     const fixture = createComponent(MatInputLabelRequiredTestComponent);
 
@@ -2324,3 +2366,29 @@ class MatInputSimple {}
   standalone: false,
 })
 class InputWithNgContainerPrefixAndSuffix {}
+
+@Component({
+  template: `
+    <mat-form-field>
+      <mat-label>Hello</mat-label>
+      <input matInput [formControl]="formControl">
+    </mat-form-field>`,
+  standalone: false,
+})
+class MatInputWithRequiredAssignableFormControl {
+  formControl = new FormControl('', [Validators.required]);
+
+  reassignFormControl() {
+    this.formControl = new FormControl();
+  }
+
+  addRequiredValidator() {
+    this.formControl.setValidators([Validators.required]);
+    this.formControl.updateValueAndValidity();
+  }
+
+  removeRequiredValidtor() {
+    this.formControl.setValidators([]);
+    this.formControl.updateValueAndValidity();
+  }
+}