fix(material/tree): Apply aria-level to all nodes

src/cdk/tree/tree.spec.ts

@@ -120,6 +120,15 @@ describe('CdkTree', () => {
         })).toBe(true);
       });
 
+      it('with the right aria-levels', () => {
+        // add a child to the first node
+        let data = dataSource.data;
+        dataSource.addChild(data[0], true);
+
+        const ariaLevels = getNodes(treeElement).map(n => n.getAttribute('aria-level'));
+        expect(ariaLevels).toEqual(['2', '3', '2', '2']);
+      });
+
       it('with the right data', () => {
         expect(dataSource.data.length).toBe(3);
 

src/cdk/tree/tree.ts

@@ -299,7 +299,7 @@ export class CdkTree<T> implements AfterContentChecked, CollectionViewer, OnDest
   exportAs: 'cdkTreeNode',
   host: {
     '[attr.aria-expanded]': 'isExpanded',
-    '[attr.aria-level]': 'role === "treeitem" ? level : null',
+    '[attr.aria-level]': 'level + 1',
     '[attr.role]': 'role',
     'class': 'cdk-tree-node',
   },
@@ -337,8 +337,7 @@ export class CdkTreeNode<T> implements FocusableOption, OnDestroy {
   }
 
   /**
-   * The role of the node should be 'group' if it's an internal node,
-   * and 'treeitem' if it's a leaf node.
+   * The role of the node should always be 'treeitem'.
    */
   @Input() role: 'treeitem' | 'group' = 'treeitem';
 
@@ -363,10 +362,10 @@ export class CdkTreeNode<T> implements FocusableOption, OnDestroy {
   focus(): void {
     this._elementRef.nativeElement.focus();
   }
-
+  
   protected _setRoleFromData(): void {
     if (this._tree.treeControl.isExpandable) {
-      this.role = this._tree.treeControl.isExpandable(this._data) ? 'group' : 'treeitem';
+      this.role = this._tree.treeControl.isExpandable(this._data) ? 'treeitem' : 'treeitem';
     } else {
       if (!this._tree.treeControl.getChildren) {
         throw getTreeControlFunctionsMissingError();
@@ -376,12 +375,12 @@ export class CdkTreeNode<T> implements FocusableOption, OnDestroy {
         this._setRoleFromChildren(childrenNodes as T[]);
       } else if (isObservable(childrenNodes)) {
         childrenNodes.pipe(takeUntil(this._destroyed))
-            .subscribe(children => this._setRoleFromChildren(children));
+          .subscribe(children => this._setRoleFromChildren(children));
       }
     }
   }
 
   protected _setRoleFromChildren(children: T[]) {
-    this.role = children && children.length ? 'group' : 'treeitem';
+    this.role = 'treeitem';
   }
 }

src/material/tree/node.ts

@@ -44,7 +44,7 @@ const _MatTreeNodeMixinBase: HasTabIndexCtor & CanDisableCtor & typeof CdkTreeNo
   inputs: ['disabled', 'tabIndex'],
   host: {
     '[attr.aria-expanded]': 'isExpanded',
-    '[attr.aria-level]': 'role === "treeitem" ? level : null',
+    '[attr.aria-level]': 'level + 1',
     '[attr.role]': 'role',
     'class': 'mat-tree-node'
   },

src/material/tree/tree.spec.ts

@@ -64,6 +64,17 @@ describe('MatTree', () => {
         });
       });
 
+      it('with the right aria-level attrs', () => {
+        // add a child to the first node
+        let data = underlyingDataSource.data;
+        underlyingDataSource.addChild(data[2]);
+        component.treeControl.expandAll();
+        fixture.detectChanges();
+
+        const ariaLevels = getNodes(treeElement).map(n => n.getAttribute('aria-level'));
+        expect(ariaLevels).toEqual(['1', '1', '1', '2']);
+      });
+
       it('with the right data', () => {
         expect(underlyingDataSource.data.length).toBe(3);