diff --git a/package-lock.json b/package-lock.json
index de991cc15d1..30e6bce4758 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -79,7 +79,7 @@
         "sizzle": "^2.3.6",
         "terser": "5.6.1",
         "tslib": "^2.1.0",
-        "typescript": "4.2.3",
+        "typescript": "4.3.5",
         "webpack": "^4.46.0",
         "ws": "7.4.6"
       },
@@ -11640,9 +11640,9 @@
       }
     },
     "node_modules/typescript": {
-      "version": "4.2.3",
-      "resolved": "https://registry.npmjs.org/typescript/-/typescript-4.2.3.tgz",
-      "integrity": "sha512-qOcYwxaByStAWrBf4x0fibwZvMRG+r4cQoTjbPtUlrWjBHbmCAww1i448U0GJ+3cNNEtebDteo/cHOR3xJ4wEw==",
+      "version": "4.3.5",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-4.3.5.tgz",
+      "integrity": "sha512-DqQgihaQ9cUrskJo9kIyW/+g0Vxsk8cDtZ52a3NGh0YNTfpUSArXSohyUGnvbPazEPLu398C0UxmKSOrPumUzA==",
       "dev": true,
       "bin": {
         "tsc": "bin/tsc",
@@ -22136,9 +22136,9 @@
       }
     },
     "typescript": {
-      "version": "4.2.3",
-      "resolved": "https://registry.npmjs.org/typescript/-/typescript-4.2.3.tgz",
-      "integrity": "sha512-qOcYwxaByStAWrBf4x0fibwZvMRG+r4cQoTjbPtUlrWjBHbmCAww1i448U0GJ+3cNNEtebDteo/cHOR3xJ4wEw==",
+      "version": "4.3.5",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-4.3.5.tgz",
+      "integrity": "sha512-DqQgihaQ9cUrskJo9kIyW/+g0Vxsk8cDtZ52a3NGh0YNTfpUSArXSohyUGnvbPazEPLu398C0UxmKSOrPumUzA==",
       "dev": true
     },
     "uglify-js": {
diff --git a/package.json b/package.json
index 4f01aebccca..57bdf895ec1 100644
--- a/package.json
+++ b/package.json
@@ -120,7 +120,7 @@
     "sizzle": "^2.3.6",
     "terser": "5.6.1",
     "tslib": "^2.1.0",
-    "typescript": "4.2.3",
+    "typescript": "4.3.5",
     "webpack": "^4.46.0",
     "ws": "7.4.6"
   },
diff --git a/src/compiler/sys/dependencies.json b/src/compiler/sys/dependencies.json
index 40cf3f70c34..333d6deecd3 100644
--- a/src/compiler/sys/dependencies.json
+++ b/src/compiler/sys/dependencies.json
@@ -50,6 +50,11 @@
         "compiler/lib.es2020.sharedmemory.d.ts",
         "compiler/lib.es2020.string.d.ts",
         "compiler/lib.es2020.symbol.wellknown.d.ts",
+        "compiler/lib.es2021.d.ts",
+        "compiler/lib.es2021.full.d.ts",
+        "compiler/lib.es2021.promise.d.ts",
+        "compiler/lib.es2021.string.d.ts",
+        "compiler/lib.es2021.weakref.d.ts",
         "compiler/lib.es5.d.ts",
         "compiler/lib.es6.d.ts",
         "compiler/lib.esnext.d.ts",
diff --git a/src/compiler/transformers/decorators-to-static/method-decorator.ts b/src/compiler/transformers/decorators-to-static/method-decorator.ts
index d0d38306030..3b71b65fda2 100644
--- a/src/compiler/transformers/decorators-to-static/method-decorator.ts
+++ b/src/compiler/transformers/decorators-to-static/method-decorator.ts
@@ -5,6 +5,7 @@ import {
   createStaticGetter,
   getAttributeTypeInfo,
   isMemberPrivate,
+  mapJSDocTagInfo,
   serializeSymbol,
   typeToString,
   validateReferences,
@@ -95,7 +96,7 @@ const parseMethodDecorator = (
     },
     docs: {
       text: ts.displayPartsToString(signature.getDocumentationComment(typeChecker)),
-      tags: signature.getJsDocTags(),
+      tags: mapJSDocTagInfo(signature.getJsDocTags()),
     },
   };
   validateReferences(diagnostics, methodMeta.complexType.references, method.type || method.name);
diff --git a/src/compiler/transformers/test/transform-utils.spec.ts b/src/compiler/transformers/test/transform-utils.spec.ts
new file mode 100644
index 00000000000..afcd0352437
--- /dev/null
+++ b/src/compiler/transformers/test/transform-utils.spec.ts
@@ -0,0 +1,41 @@
+import { mapJSDocTagInfo } from '../transform-utils';
+
+describe('transform utils', () => {
+  it('flattens TypeScript JSDocTagInfo to Stencil JSDocTagInfo', () => {
+    // tags corresponds to the following JSDoc
+    /*
+     * @param foo the first parameter
+     * @param bar
+     * @returns
+     * @see {@link https://example.com}
+     */
+    const tags = [
+      {
+        name: 'param',
+        text: [
+          { text: 'foo', kind: 'parameterName' },
+          { text: ' ', kind: 'space' },
+          { text: 'the first parameter', kind: 'text' },
+        ],
+      },
+      { name: 'param', text: [{ text: 'bar', kind: 'text' }] },
+      { name: 'returns', text: undefined },
+      {
+        name: 'see',
+        text: [
+          { text: '', kind: 'text' },
+          { text: '{@link ', kind: 'link' },
+          { text: 'https://example.com', kind: 'linkText' },
+          { text: '}', kind: 'link' },
+        ],
+      },
+    ];
+
+    expect(mapJSDocTagInfo(tags)).toEqual([
+      { name: 'param', text: 'foo the first parameter' },
+      { name: 'param', text: 'bar' },
+      { name: 'returns', text: undefined },
+      { name: 'see', text: '{@link https://example.com}' },
+    ]);
+  });
+});
diff --git a/src/compiler/transformers/transform-utils.ts b/src/compiler/transformers/transform-utils.ts
index ce69f21c0b4..3f42b9ea0c1 100644
--- a/src/compiler/transformers/transform-utils.ts
+++ b/src/compiler/transformers/transform-utils.ts
@@ -515,11 +515,23 @@ export const serializeSymbol = (checker: ts.TypeChecker, symbol: ts.Symbol): d.C
     };
   }
   return {
-    tags: symbol.getJsDocTags().map((tag) => ({ text: tag.text, name: tag.name })),
+    tags: mapJSDocTagInfo(symbol.getJsDocTags()),
     text: ts.displayPartsToString(symbol.getDocumentationComment(checker)),
   };
 };
 
+/**
+ * Maps a TypeScript 4.3+ JSDocTagInfo to a flattened Stencil CompilerJsDocTagInfo.
+ * @param tags A readonly array of JSDocTagInfo objects.
+ * @returns An array of CompilerJsDocTagInfo objects.
+ */
+export const mapJSDocTagInfo = (tags: readonly ts.JSDocTagInfo[]): d.CompilerJsDocTagInfo[] => {
+  // The text following a tag is split semantically by TS 4.3+, e.g. '@param foo the first parameter' ->
+  // [{text: 'foo', kind: 'parameterName'}, {text: ' ', kind: 'space'}, {text: 'the first parameter', kind: 'text'}], so
+  // we join the elements to reconstruct the text.
+  return tags.map((tag) => ({ ...tag, text: tag.text?.map((part) => part.text).join('') }));
+};
+
 export const serializeDocsSymbol = (checker: ts.TypeChecker, symbol: ts.Symbol) => {
   const type = checker.getTypeOfSymbolAtLocation(symbol, symbol.valueDeclaration);
   const set = new Set<string>();
diff --git a/src/mock-doc/comment-node.ts b/src/mock-doc/comment-node.ts
index a3451ff0d1d..2f14cfa5342 100644
--- a/src/mock-doc/comment-node.ts
+++ b/src/mock-doc/comment-node.ts
@@ -6,15 +6,15 @@ export class MockComment extends MockNode {
     super(ownerDocument, NODE_TYPES.COMMENT_NODE, NODE_NAMES.COMMENT_NODE, data);
   }
 
-  cloneNode(_deep?: boolean) {
+  override cloneNode(_deep?: boolean) {
     return new MockComment(null, this.nodeValue);
   }
 
-  get textContent() {
+  override get textContent() {
     return this.nodeValue;
   }
 
-  set textContent(text) {
+  override set textContent(text) {
     this.nodeValue = text;
   }
 }
diff --git a/src/mock-doc/document-fragment.ts b/src/mock-doc/document-fragment.ts
index 5f5a5039906..759865b7902 100644
--- a/src/mock-doc/document-fragment.ts
+++ b/src/mock-doc/document-fragment.ts
@@ -13,7 +13,7 @@ export class MockDocumentFragment extends MockHTMLElement {
     return getElementById(this, id);
   }
 
-  cloneNode(deep?: boolean) {
+  override cloneNode(deep?: boolean) {
     const cloned = new MockDocumentFragment(null);
 
     if (deep) {
diff --git a/src/mock-doc/document.ts b/src/mock-doc/document.ts
index adbdd16dbac..f5495fbd854 100644
--- a/src/mock-doc/document.ts
+++ b/src/mock-doc/document.ts
@@ -42,10 +42,10 @@ export class MockDocument extends MockHTMLElement {
     }
   }
 
-  get dir() {
+  override get dir() {
     return this.documentElement.dir;
   }
-  set dir(value: string) {
+  override set dir(value: string) {
     this.documentElement.dir = value;
   }
 
@@ -166,7 +166,7 @@ export class MockDocument extends MockHTMLElement {
     }
   }
 
-  appendChild(newNode: MockElement) {
+  override appendChild(newNode: MockElement) {
     newNode.remove();
     newNode.parentNode = this;
     this.childNodes.push(newNode);
@@ -222,14 +222,14 @@ export class MockDocument extends MockHTMLElement {
     return getElementsByName(this, elmName.toLowerCase());
   }
 
-  get title() {
+  override get title() {
     const title = this.head.childNodes.find((elm) => elm.nodeName === 'TITLE') as MockElement;
     if (title != null && typeof title.textContent === 'string') {
       return title.textContent.trim();
     }
     return '';
   }
-  set title(value: string) {
+  override set title(value: string) {
     const head = this.head;
     let title = head.childNodes.find((elm) => elm.nodeName === 'TITLE') as MockElement;
     if (title == null) {
diff --git a/src/mock-doc/element.ts b/src/mock-doc/element.ts
index 1dac29fcce4..c91158ac110 100644
--- a/src/mock-doc/element.ts
+++ b/src/mock-doc/element.ts
@@ -107,10 +107,10 @@ export class MockImageElement extends MockHTMLElement {
     super(ownerDocument, 'img');
   }
 
-  get draggable() {
+  override get draggable() {
     return this.getAttributeNS(null, 'draggable') !== 'false';
   }
-  set draggable(value: boolean) {
+  override set draggable(value: boolean) {
     this.setAttributeNS(null, 'draggable', value);
   }
 
@@ -243,24 +243,24 @@ export class MockStyleElement extends MockHTMLElement {
     this.sheet = new MockCSSStyleSheet(this);
   }
 
-  get innerHTML() {
+  override get innerHTML() {
     return getStyleElementText(this);
   }
-  set innerHTML(value: string) {
+  override set innerHTML(value: string) {
     setStyleElementText(this, value);
   }
 
-  get innerText() {
+  override get innerText() {
     return getStyleElementText(this);
   }
-  set innerText(value: string) {
+  override set innerText(value: string) {
     setStyleElementText(this, value);
   }
 
-  get textContent() {
+  override get textContent() {
     return getStyleElementText(this);
   }
-  set textContent(value: string) {
+  override set textContent(value: string) {
     setStyleElementText(this, value);
   }
 }
@@ -318,14 +318,14 @@ export class MockTemplateElement extends MockHTMLElement {
     this.content = new MockDocumentFragment(ownerDocument);
   }
 
-  get innerHTML() {
+  override get innerHTML() {
     return this.content.innerHTML;
   }
-  set innerHTML(html: string) {
+  override set innerHTML(html: string) {
     this.content.innerHTML = html;
   }
 
-  cloneNode(deep?: boolean) {
+  override cloneNode(deep?: boolean) {
     const cloned = new MockTemplateElement(null);
     cloned.attributes = cloneAttributes(this.attributes);
 
diff --git a/src/mock-doc/node.ts b/src/mock-doc/node.ts
index dd2ad7fb69e..3617e7a605b 100644
--- a/src/mock-doc/node.ts
+++ b/src/mock-doc/node.ts
@@ -66,7 +66,7 @@ export class MockNode {
     return -1;
   }
 
-  get firstChild() {
+  get firstChild(): MockNode | null {
     return this.childNodes[0] || null;
   }
 
@@ -103,11 +103,11 @@ export class MockNode {
     return this === node;
   }
 
-  get lastChild() {
+  get lastChild(): MockNode | null {
     return this.childNodes[this.childNodes.length - 1] || null;
   }
 
-  get nextSibling() {
+  get nextSibling(): MockNode | null {
     if (this.parentNode != null) {
       const index = this.parentNode.childNodes.indexOf(this) + 1;
       return this.parentNode.childNodes[index] || null;
@@ -129,7 +129,7 @@ export class MockNode {
     this.parentNode = value;
   }
 
-  get previousSibling() {
+  get previousSibling(): MockNode | null {
     if (this.parentNode != null) {
       const index = this.parentNode.childNodes.indexOf(this) - 1;
       return this.parentNode.childNodes[index] || null;
@@ -273,7 +273,7 @@ export class MockElement extends MockNode {
     dispatchEvent(this, new MockEvent('click', { bubbles: true, cancelable: true, composed: true }));
   }
 
-  cloneNode(_deep?: boolean): MockElement {
+  override cloneNode(_deep?: boolean): MockElement {
     // implemented on MockElement.prototype from within element.ts
     return null;
   }
@@ -304,7 +304,7 @@ export class MockElement extends MockNode {
     return dispatchEvent(this, ev);
   }
 
-  get firstElementChild() {
+  get firstElementChild(): MockElement | null {
     return this.children[0] || null;
   }
 
@@ -481,7 +481,7 @@ export class MockElement extends MockNode {
     this.setAttributeNS(null, 'lang', value);
   }
 
-  get lastElementChild() {
+  get lastElementChild(): MockElement | null {
     const children = this.children;
     return children[children.length - 1] || null;
   }
@@ -670,12 +670,12 @@ export class MockElement extends MockNode {
     this.nodeName = value;
   }
 
-  get textContent() {
+  override get textContent() {
     const text: string[] = [];
     getTextContent(this.childNodes, text);
     return text.join('');
   }
-  set textContent(value: string) {
+  override set textContent(value: string) {
     setTextContent(this, value);
   }
 
@@ -951,7 +951,7 @@ export class MockElement extends MockNode {
     /**/
   }
 
-  toString(opts?: SerializeNodeToHtmlOptions) {
+  override toString(opts?: SerializeNodeToHtmlOptions) {
     return serializeNodeToHtml(this as any, opts);
   }
 }
@@ -1011,26 +1011,26 @@ function insertBefore(parentNode: MockNode, newNode: MockNode, referenceNode: Mo
 }
 
 export class MockHTMLElement extends MockElement {
-  namespaceURI = 'http://www.w3.org/1999/xhtml';
+  override namespaceURI = 'http://www.w3.org/1999/xhtml';
 
   constructor(ownerDocument: any, nodeName: string) {
     super(ownerDocument, typeof nodeName === 'string' ? nodeName.toUpperCase() : null);
   }
 
-  get tagName() {
+  override get tagName() {
     return this.nodeName;
   }
-  set tagName(value: string) {
+  override set tagName(value: string) {
     this.nodeName = value;
   }
 
-  get attributes() {
+  override get attributes() {
     if (this.__attributeMap == null) {
       this.__attributeMap = createAttributeProxy(true);
     }
     return this.__attributeMap;
   }
-  set attributes(attrs: MockAttributeMap) {
+  override set attributes(attrs: MockAttributeMap) {
     this.__attributeMap = attrs;
   }
 }
@@ -1040,14 +1040,14 @@ export class MockTextNode extends MockNode {
     super(ownerDocument, NODE_TYPES.TEXT_NODE, NODE_NAMES.TEXT_NODE, text);
   }
 
-  cloneNode(_deep?: boolean) {
+  override cloneNode(_deep?: boolean) {
     return new MockTextNode(null, this.nodeValue);
   }
 
-  get textContent() {
+  override get textContent() {
     return this.nodeValue;
   }
-  set textContent(text) {
+  override set textContent(text) {
     this.nodeValue = text;
   }
 
diff --git a/src/screenshot/connector-local.ts b/src/screenshot/connector-local.ts
index 89b59742859..2b839e35569 100644
--- a/src/screenshot/connector-local.ts
+++ b/src/screenshot/connector-local.ts
@@ -5,7 +5,7 @@ import { normalizePath } from '@utils';
 import { ScreenshotConnector } from './connector-base';
 
 export class ScreenshotLocalConnector extends ScreenshotConnector {
-  async publishBuild(results: d.ScreenshotBuildResults) {
+  override async publishBuild(results: d.ScreenshotBuildResults) {
     if (this.updateMaster || !results.masterBuild) {
       results.masterBuild = {
         id: 'master',
@@ -64,7 +64,7 @@ export class ScreenshotLocalConnector extends ScreenshotConnector {
     return results;
   }
 
-  async getScreenshotCache() {
+  override async getScreenshotCache() {
     let screenshotCache: d.ScreenshotCache = null;
 
     try {
@@ -74,7 +74,7 @@ export class ScreenshotLocalConnector extends ScreenshotConnector {
     return screenshotCache;
   }
 
-  async updateScreenshotCache(cache: d.ScreenshotCache, buildResults: d.ScreenshotBuildResults) {
+  override async updateScreenshotCache(cache: d.ScreenshotCache, buildResults: d.ScreenshotBuildResults) {
     cache = await super.updateScreenshotCache(cache, buildResults);
 
     await writeFile(this.screenshotCacheFilePath, JSON.stringify(cache, null, 2));
@@ -100,7 +100,7 @@ function createLocalCompareApp(
   <meta http-equiv="x-ua-compatible" content="IE=Edge">
   <link href="${appSrcUrl}/build/app.css" rel="stylesheet">
   <script type="module" src="${appSrcUrl}/build/app.esm.js"></script>
-  <script nomodule src="${appSrcUrl}/build/app.js"></script>  
+  <script nomodule src="${appSrcUrl}/build/app.js"></script>
   <link rel="icon" type="image/x-icon" href="${appSrcUrl}/assets/favicon.ico">
 </head>
 <body>
diff --git a/src/testing/mock-fetch.ts b/src/testing/mock-fetch.ts
index bf3d57d84a0..6b7306b99ee 100644
--- a/src/testing/mock-fetch.ts
+++ b/src/testing/mock-fetch.ts
@@ -133,9 +133,9 @@ export const mockFetch = {
 };
 
 class MockResponse404 extends MockResponse {
-  ok = false;
-  status = 404;
-  statusText = 'Not Found';
+  override ok = false;
+  override status = 404;
+  override statusText = 'Not Found';
   constructor() {
     super('', {
       headers: new MockHeaders({
@@ -143,10 +143,10 @@ class MockResponse404 extends MockResponse {
       }),
     });
   }
-  async json() {
+  override async json() {
     return { status: 404, statusText: 'Not Found' };
   }
-  async text() {
+  override async text() {
     return 'Not Found';
   }
 }
diff --git a/src/testing/mocks.ts b/src/testing/mocks.ts
index a152d10b176..197d5897dfe 100644
--- a/src/testing/mocks.ts
+++ b/src/testing/mocks.ts
@@ -137,12 +137,7 @@ export function mockLogger() {
   return new TestingLogger();
 }
 
-export interface TestingSystem extends CompilerSystem {
-  diskReads: number;
-  diskWrites: number;
-}
-
-export function mockStencilSystem(): TestingSystem {
+export function mockStencilSystem(): CompilerSystem {
   return createTestingSystem();
 }
 
diff --git a/src/testing/puppeteer/puppeteer-element.ts b/src/testing/puppeteer/puppeteer-element.ts
index 53ade2cbe90..7582376396e 100644
--- a/src/testing/puppeteer/puppeteer-element.ts
+++ b/src/testing/puppeteer/puppeteer-element.ts
@@ -50,7 +50,7 @@ export class E2EElement extends MockHTMLElement implements pd.E2EElementInternal
     return eventSpy;
   }
 
-  async click(options?: puppeteer.ClickOptions) {
+  override async click(options?: puppeteer.ClickOptions) {
     await this._elmHandle.click(options);
     await this._page.waitForChanges();
   }
@@ -186,19 +186,19 @@ export class E2EElement extends MockHTMLElement implements pd.E2EElementInternal
     });
   }
 
-  getAttribute(name: string) {
+  override getAttribute(name: string) {
     this._validate();
     return super.getAttribute(name);
   }
 
-  setAttribute(name: string, value: any) {
+  override setAttribute(name: string, value: any) {
     this._queueAction({
       setAttributeName: name,
       setAttributeValue: value,
     });
   }
 
-  removeAttribute(name: string) {
+  override removeAttribute(name: string) {
     this._queueAction({
       removeAttribute: name,
     });
@@ -211,7 +211,7 @@ export class E2EElement extends MockHTMLElement implements pd.E2EElementInternal
     });
   }
 
-  get classList() {
+  override get classList() {
     const api: any = {
       add: (...classNames: string[]) => {
         classNames.forEach((className) => {
@@ -240,60 +240,60 @@ export class E2EElement extends MockHTMLElement implements pd.E2EElementInternal
     return api;
   }
 
-  get className() {
+  override get className() {
     this._validate();
     return super.className;
   }
 
-  set className(value: string) {
+  override set className(value: string) {
     this._queueAction({
       setPropertyName: 'className',
       setPropertyValue: value,
     });
   }
 
-  get id() {
+  override get id() {
     this._validate();
     return super.id;
   }
 
-  set id(value: string) {
+  override set id(value: string) {
     this._queueAction({
       setPropertyName: 'id',
       setPropertyValue: value,
     });
   }
 
-  get innerHTML() {
+  override get innerHTML() {
     this._validate();
     return super.innerHTML;
   }
 
-  set innerHTML(value: string) {
+  override set innerHTML(value: string) {
     this._queueAction({
       setPropertyName: 'innerHTML',
       setPropertyValue: value,
     });
   }
 
-  get innerText() {
+  override get innerText() {
     this._validate();
     return super.innerText;
   }
 
-  set innerText(value: string) {
+  override set innerText(value: string) {
     this._queueAction({
       setPropertyName: 'innerText',
       setPropertyValue: value,
     });
   }
 
-  get nodeValue() {
+  override get nodeValue() {
     this._validate();
     return super.nodeValue;
   }
 
-  set nodeValue(value: string) {
+  override set nodeValue(value: string) {
     if (typeof value === 'string') {
       this._queueAction({
         setPropertyName: 'nodeValue',
@@ -302,54 +302,54 @@ export class E2EElement extends MockHTMLElement implements pd.E2EElementInternal
     }
   }
 
-  get outerHTML() {
+  override get outerHTML() {
     this._validate();
     return super.outerHTML;
   }
 
-  set outerHTML(_: any) {
+  override set outerHTML(_: any) {
     throw new Error(`outerHTML is read-only`);
   }
 
-  get shadowRoot() {
+  override get shadowRoot() {
     this._validate();
     return super.shadowRoot;
   }
 
-  set shadowRoot(value: any) {
+  override set shadowRoot(value: any) {
     super.shadowRoot = value;
   }
 
-  get tabIndex() {
+  override get tabIndex() {
     this._validate();
     return super.tabIndex;
   }
 
-  set tabIndex(value: number) {
+  override set tabIndex(value: number) {
     this._queueAction({
       setPropertyName: 'tabIndex',
       setPropertyValue: value,
     });
   }
 
-  get textContent() {
+  override get textContent() {
     this._validate();
     return super.textContent;
   }
 
-  set textContent(value: string) {
+  override set textContent(value: string) {
     this._queueAction({
       setPropertyName: 'textContent',
       setPropertyValue: value,
     });
   }
 
-  get title() {
+  override get title() {
     this._validate();
     return super.title;
   }
 
-  set title(value: string) {
+  override set title(value: string) {
     this._queueAction({
       setPropertyName: 'title',
       setPropertyValue: value,
diff --git a/src/testing/testing-sys.ts b/src/testing/testing-sys.ts
index 0a4c54d0c6c..08a68a53ec6 100644
--- a/src/testing/testing-sys.ts
+++ b/src/testing/testing-sys.ts
@@ -1,8 +1,18 @@
+import type { CompilerSystem } from '@stencil/core/internal';
 import { createSystem } from '../compiler/sys/stencil-sys';
 import { createHash } from 'crypto';
 import path from 'path';
 
-export const createTestingSystem = () => {
+export interface TestingSystem extends CompilerSystem {
+  diskReads: number;
+  diskWrites: number;
+}
+
+function isTestingSystem(sys: CompilerSystem): sys is TestingSystem {
+  return 'diskReads' in sys && 'diskWrites' in sys;
+}
+
+export const createTestingSystem = (): TestingSystem => {
   let diskReads = 0;
   let diskWrites = 0;
   const sys = createSystem();
@@ -52,7 +62,7 @@ export const createTestingSystem = () => {
   sys.writeFile = wrapWrite(sys.writeFile);
   sys.writeFileSync = wrapWrite(sys.writeFileSync);
 
-  return Object.defineProperties(sys, {
+  Object.defineProperties(sys, {
     diskReads: {
       get() {
         return diskReads;
@@ -70,4 +80,10 @@ export const createTestingSystem = () => {
       },
     },
   });
+
+  if (!isTestingSystem(sys)) {
+    throw new Error('could not generate TestingSystem');
+  }
+
+  return sys;
 };
diff --git a/tsconfig.json b/tsconfig.json
index b72f10245a6..1e0c7ee1bba 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -13,6 +13,7 @@
     "module": "esnext",
     "moduleResolution": "node",
     "noImplicitAny": true,
+    "noImplicitOverride": true,
     "noImplicitReturns": true,
     "noUnusedLocals": true,
     "noUnusedParameters": true,