fix(@angular-devkit/schematics): Fix merge that causes an overwrite

This fixes #11337 to allow for merging of a tree with another when the the file already exists in the tree being merged into.

packages/angular_devkit/schematics/src/tree/host-tree.ts

@@ -56,7 +56,7 @@ export class HostDirEntry implements DirEntry {
     readonly path: Path,
     protected _host: virtualFs.SyncDelegateHost,
     protected _tree: Tree,
-  ) {}
+  ) { }
 
   get subdirs(): PathFragment[] {
     return this._host.list(this.path)
@@ -180,7 +180,7 @@ export class HostTree implements Tree {
         case 'c': {
           const { path, content } = action;
 
-          if ((this._willCreate(path) || this._willOverwrite(path))) {
+          if (this._willCreate(path) || this._willOverwrite(path) || this.exists(path)) {
             const existingContent = this.read(path);
             if (existingContent && content.equals(existingContent)) {
               // Identical outcome; no action required

packages/angular_devkit/schematics/src/tree/host-tree_spec.ts

@@ -7,9 +7,47 @@
  */
 import { normalize, virtualFs } from '@angular-devkit/core';
 import { FilterHostTree, HostTree } from './host-tree';
+import { MergeStrategy } from './interface';
 
 describe('HostTree', () => {
+  describe('merge', () => {
+    it('should create files from each tree', () => {
+      const tree = new HostTree();
+      tree.create('/file1', 'a');
+      const tree2 = new HostTree();
+      tree2.create('/file2', 'a');
+      tree.merge(tree2);
+      expect(tree.actions[0].kind).toEqual('c');
+      expect(tree.actions[1].kind).toEqual('c');
+    });
+
+    it('should overwrite if the file exists in one tree', () => {
+      const tree = new HostTree();
+      tree.create('/file1', 'a');
+      const tree2 = new HostTree();
+      tree2.create('/file1', 'b');
+      tree.merge(tree2, MergeStrategy.Overwrite);
+      expect(tree.actions[0].kind).toEqual('c');
+    });
 
+    it('should throw if the file exists in one tree with the correct MergeStrategy', () => {
+      const tree = new HostTree();
+      tree.create('/file1', 'a');
+      const tree2 = new HostTree();
+      tree2.create('/file1', 'b');
+      expect(() => tree.merge(tree2)).toThrow();
+    });
+
+    it('should not have a second action if the file content is the same', () => {
+      const tree = new HostTree();
+      tree.create('/file1', 'a');
+      const tree2 = new HostTree();
+      tree2.create('/file1', 'a');
+      tree.merge(tree2, MergeStrategy.Overwrite);
+      expect(tree.actions[0].kind).toEqual('c');
+      expect(tree.actions.length).toEqual(1);
+    });
+  });
 });
 
 describe('FilterHostTree', () => {

tests/legacy-cli/e2e/tests/generate/component/component-duplicate.ts

@@ -1,6 +1,7 @@
+import { oneLine } from 'common-tags';
+import { appendToFile } from '../../../utils/fs';
 import { ng } from '../../../utils/process';
 import { expectToFail } from '../../../utils/utils';
-import { oneLine } from 'common-tags';
 
 export default function () {
   return ng('generate', 'component', 'test-component')
@@ -12,5 +13,6 @@ export default function () {
           in ${output.stdout}.`);
       }
     })
+    .then(() => appendToFile('src/app/test-component/test-component.component.ts', '\n// new content'))
     .then(() => expectToFail(() => ng('generate', 'component', 'test-component')));
 }