fix(material/radio): required attribute being set on buttons

fixes when `MatRadioGroup` is set to be required it was setting all of its `MatRadioButton'
to be required as-well which is confusing for assistive technologies, this commit ensures
we only set aria-required on group rather than all buttons unless button is being used
standalone of `MatRadioGroup`

fixes angular#30399

Author: naaajii

src/material/radio/radio.html

@@ -2,13 +2,17 @@
   <div class="mdc-radio" [class.mdc-radio--disabled]="disabled">
     <!-- Render this element first so the input is on top. -->
     <div class="mat-mdc-radio-touch-target" (click)="_onTouchTargetClick($event)"></div>
+    <!-- We don't want to add required attribute when button is used with MatRadioGroup as
+    we want to set directly it on MatRadioGroup as setting it on all MatRadioButton would be 
+    confusing for assistive technology users unless MatRadioButton is being used 
+    standalone -->
     <input #input class="mdc-radio__native-control" type="radio"
            [id]="inputId"
            [checked]="checked"
            [disabled]="disabled && !disabledInteractive"
            [attr.name]="name"
            [attr.value]="value"
-           [required]="required"
+           [required]="radioGroup ? null : required" 
            [attr.aria-label]="ariaLabel"
            [attr.aria-labelledby]="ariaLabelledby"
            [attr.aria-describedby]="ariaDescribedby"

src/material/radio/radio.spec.ts

@@ -524,6 +524,33 @@ describe('MatRadio', () => {
       expect(groupInstance.selected).toBe(null);
       expect(groupInstance.value).toBe('fire');
     });
+
+    it('should set aria-required on radio group when required', () => {
+      fixture.changeDetectorRef.markForCheck();
+      fixture.detectChanges();
+
+      let group = fixture.debugElement.query(By.directive(MatRadioGroup)).nativeElement;
+
+      // by default it shouldn't be there
+      expect(group.hasAttribute('aria-required')).toBeFalse();
+
+      testComponent.isGroupRequired = true;
+      fixture.changeDetectorRef.markForCheck();
+      fixture.detectChanges();
+
+      group = fixture.debugElement.query(By.directive(MatRadioGroup)).nativeElement;
+      expect(group.hasAttribute('aria-required')).toBe(true);
+    });
+
+    it('should set not set required attribute on matRadioButton when matRadioGroup is required', () => {
+      testComponent.isGroupRequired = true;
+      fixture.changeDetectorRef.markForCheck();
+      fixture.detectChanges();
+
+      for (const radio of radioDebugElements) {
+        expect(radio.nativeElement.hasAttribute('aria-required')).toBeFalse();
+      }
+    });
   });
 
   describe('group with ngModel', () => {

src/material/radio/radio.ts

@@ -118,6 +118,7 @@ export function MAT_RADIO_DEFAULT_OPTIONS_FACTORY(): MatRadioDefaultOptions {
   host: {
     'role': 'radiogroup',
     'class': 'mat-mdc-radio-group',
+    '[attr.aria-required]': 'required ? required : null',
   },
 })
 export class MatRadioGroup implements AfterContentInit, OnDestroy, ControlValueAccessor {