fix: prevent nested expansion panels from registering to the same accordion

src/cdk/accordion/accordion-item.ts

@@ -14,6 +14,7 @@ import {
   OnDestroy,
   Optional,
   ChangeDetectorRef,
+  SkipSelf,
 } from '@angular/core';
 import {UniqueSelectionDispatcher} from '@angular/cdk/collections';
 import {CdkAccordion} from './accordion';
@@ -30,6 +31,11 @@ let nextId = 0;
 @Directive({
   selector: 'cdk-accordion-item, [cdkAccordionItem]',
   exportAs: 'cdkAccordionItem',
+  providers: [
+    // Provide CdkAccordion as undefined to prevent nested accordion items from registering
+    // to the same accordion.
+    {provide: CdkAccordion, useValue: undefined},
+  ],
 })
 export class CdkAccordionItem implements OnDestroy {
   /** Subscription to openAll/closeAll events. */
@@ -90,7 +96,7 @@ export class CdkAccordionItem implements OnDestroy {
   /** Unregister function for _expansionDispatcher. */
   private _removeUniqueSelectionListener: () => void = () => {};
 
-  constructor(@Optional() public accordion: CdkAccordion,
+  constructor(@Optional() @SkipSelf() public accordion: CdkAccordion,
               private _changeDetectorRef: ChangeDetectorRef,
               protected _expansionDispatcher: UniqueSelectionDispatcher) {
     this._removeUniqueSelectionListener =

src/cdk/accordion/accordion.spec.ts

@@ -12,7 +12,8 @@ describe('CdkAccordion', () => {
         CdkAccordionModule
       ],
       declarations: [
-        SetOfItems
+        SetOfItems,
+        NestedItems,
       ],
     });
     TestBed.compileComponents();
@@ -53,6 +54,14 @@ describe('CdkAccordion', () => {
     expect(firstPanel.expanded).toBeTruthy();
     expect(secondPanel.expanded).toBeTruthy();
   });
+
+  it('should not register nested items to the same accordion', () => {
+    const fixture = TestBed.createComponent(NestedItems);
+    const innerItem = fixture.componentInstance.innerItem;
+    const outerItem = fixture.componentInstance.outerItem;
+
+    expect(innerItem.accordion).not.toBe(outerItem.accordion);
+  });
 });
 
 @Component({template: `
@@ -65,3 +74,15 @@ class SetOfItems {
   @ViewChild('item2') item2;
   multi: boolean = false;
 }
+
+
+@Component({template: `
+  <cdk-accordion>
+    <cdk-accordion-item #outerItem="cdkAccordionItem">
+      <cdk-accordion-item #innerItem="cdkAccordionItem"></cdk-accordion-item>
+    </cdk-accordion-item>
+  </cdk-accordion>`})
+class NestedItems {
+  @ViewChild('outerItem') outerItem: CdkAccordionItem;
+  @ViewChild('innerItem') innerItem: CdkAccordionItem;
+}

src/lib/expansion/accordion.spec.ts

@@ -2,18 +2,19 @@ import {async, TestBed} from '@angular/core/testing';
 import {Component, ViewChild} from '@angular/core';
 import {By} from '@angular/platform-browser';
 import {BrowserAnimationsModule} from '@angular/platform-browser/animations';
-import {MatExpansionModule, MatAccordion} from './index';
+import {MatExpansionModule, MatAccordion, MatExpansionPanel} from './index';
 
 
-describe('CdkAccordion', () => {
+describe('MatAccordion', () => {
   beforeEach(async(() => {
     TestBed.configureTestingModule({
       imports: [
         BrowserAnimationsModule,
         MatExpansionModule
       ],
       declarations: [
-        SetOfItems
+        NestedPanel,
+        SetOfItems,
       ],
     });
     TestBed.compileComponents();
@@ -84,6 +85,14 @@ describe('CdkAccordion', () => {
     expect(panels[0].classes['mat-expanded']).toBeFalsy();
     expect(panels[1].classes['mat-expanded']).toBeFalsy();
   });
+
+  it('should not register nested panels to the same accordion', () => {
+    const fixture = TestBed.createComponent(NestedPanel);
+    const innerPanel = fixture.componentInstance.innerPanel;
+    const outerPanel = fixture.componentInstance.outerPanel;
+
+    expect(innerPanel.accordion).not.toBe(outerPanel.accordion);
+  });
 });
 
 
@@ -106,3 +115,18 @@ class SetOfItems {
   secondPanelExpanded: boolean = false;
   secondPanelDisabled: boolean = false;
 }
+
+@Component({template: `
+  <mat-accordion>
+    <mat-expansion-panel #outerPanel="matExpansionPanel">
+      <mat-expansion-panel-header>Outer Panel</mat-expansion-panel-header>
+      <mat-expansion-panel #innerPanel="matExpansionPanel">
+        <mat-expansion-panel-header>Inner Panel</mat-expansion-panel-header>
+        <p>Content</p>
+      </mat-expansion-panel>
+    </mat-expansion-panel>
+  </mat-accordion>`})
+class NestedPanel {
+  @ViewChild('outerPanel') outerPanel: MatExpansionPanel;
+  @ViewChild('innerPanel') innerPanel: MatExpansionPanel;
+}

src/lib/expansion/expansion-panel.ts

@@ -23,6 +23,7 @@ import {
   OnDestroy,
   Optional,
   SimpleChanges,
+  SkipSelf,
   ViewContainerRef,
   ViewEncapsulation,
 } from '@angular/core';
@@ -56,6 +57,11 @@ let uniqueId = 0;
   inputs: ['disabled', 'expanded'],
   outputs: ['opened', 'closed', 'expandedChange'],
   animations: [matExpansionAnimations.bodyExpansion],
+  providers: [
+    // Provide MatAccordion as undefined to prevent nested expansion panels from registering
+    // to the same accordion.
+    {provide: MatAccordion, useValue: undefined},
+  ],
   host: {
     'class': 'mat-expansion-panel',
     '[class.mat-expanded]': 'expanded',
@@ -87,7 +93,7 @@ export class MatExpansionPanel extends CdkAccordionItem
   /** ID for the associated header element. Used for a11y labelling. */
   _headerId = `mat-expansion-panel-header-${uniqueId++}`;
 
-  constructor(@Optional() accordion: MatAccordion,
+  constructor(@Optional() @SkipSelf() accordion: MatAccordion,
               _changeDetectorRef: ChangeDetectorRef,
               _uniqueSelectionDispatcher: UniqueSelectionDispatcher,
               private _viewContainerRef: ViewContainerRef) {