fix(@angular-devkit/schematics): support delegating trees in the filter rule

`filter()` only accepted a `HostTree` and threw "Tree type is not supported." for any other tree.
A `UnitTestTree` is a `DelegateTree` around a `HostTree`, so a schematic that uses `filter()` failed
in unit tests when run on the tree returned by a previous `runSchematic` call. Unwrap delegating
trees and filter the `HostTree` they wrap.

Closes #26000

Author: Thanatos9404

packages/angular_devkit/schematics/src/rules/base.ts

@@ -9,6 +9,7 @@
 import { concat, lastValueFrom, map, mapTo, toArray } from 'rxjs';
 import { FileOperator, Rule, Source } from '../engine/interface';
 import { SchematicsException } from '../exception/exception';
+import { DelegateTree } from '../tree/delegate';
 import { FilterHostTree, HostTree } from '../tree/host-tree';
 import { FileEntry, FilePredicate, MergeStrategy, Tree } from '../tree/interface';
 import { ScopedTree } from '../tree/scoped';
@@ -74,8 +75,15 @@ export function noop(): Rule {
 
 export function filter(predicate: FilePredicate<boolean>): Rule {
   return (tree: Tree) => {
-    if (HostTree.isHostTree(tree)) {
-      return new FilterHostTree(tree, predicate);
+    // Trees such as `UnitTestTree` delegate to a `HostTree`, so filter the tree they wrap.
+    let hostTree = tree;
+    while (hostTree instanceof DelegateTree) {
+      // cast to allow access
+      hostTree = (hostTree as unknown as { _other: Tree })._other;
+    }
+
+    if (HostTree.isHostTree(hostTree)) {
+      return new FilterHostTree(hostTree, predicate);
     } else {
       throw new SchematicsException('Tree type is not supported.');
     }

packages/angular_devkit/schematics/src/rules/base_spec.ts

@@ -8,11 +8,12 @@
 
 import { Path, virtualFs } from '@angular-devkit/core';
 import { lastValueFrom, of as observableOf } from 'rxjs';
+import { UnitTestTree } from '../../testing';
 import { Rule, SchematicContext, Source } from '../engine/interface';
 import { HostTree } from '../tree/host-tree';
 import { MergeStrategy, Tree } from '../tree/interface';
 import { empty } from '../tree/static';
-import { apply, applyToSubtree, chain, partitionApplyMerge } from './base';
+import { apply, applyToSubtree, chain, filter, partitionApplyMerge } from './base';
 import { callRule, callSource } from './call';
 import { move } from './move';
 
@@ -242,3 +243,37 @@ describe('applyToSubtree', () => {
     expect(result.exists('a/c/file3')).toBe(true);
   });
 });
+
+describe('filter', () => {
+  it('works with a HostTree', async () => {
+    const tree = new HostTree();
+    tree.create('/file1.ts', '');
+    tree.create('/file2.js', '');
+
+    const result = await lastValueFrom(
+      callRule(
+        filter((path) => path.endsWith('.ts')),
+        observableOf(tree),
+        context,
+      ),
+    );
+    expect(result.exists('/file1.ts')).toBe(true);
+    expect(result.exists('/file2.js')).toBe(false);
+  });
+
+  it('works with a UnitTestTree', async () => {
+    const tree = new HostTree();
+    tree.create('/file1.ts', '');
+    tree.create('/file2.js', '');
+
+    const result = await lastValueFrom(
+      callRule(
+        filter((path) => path.endsWith('.ts')),
+        observableOf(new UnitTestTree(new UnitTestTree(tree))),
+        context,
+      ),
+    );
+    expect(result.exists('/file1.ts')).toBe(true);
+    expect(result.exists('/file2.js')).toBe(false);
+  });
+});