fix: skip dangerouslySetInnerHtml hydration warning if it's undefined (#18676)

* test: Add failing case for dangerouslySetInnerHtml=undefined

* fix: skip dangerouslySetInnerHtml warning if it's undefined

* test: add similar test that should trigger the warning

* chore: Remove redundant nullish check

* Poke yarn_test_www_variant which timed out

* test: Add smaller test for innerHTML=string to innerHTML=undefined

packages/react-dom/src/__tests__/ReactServerRenderingHydration-test.js

@@ -548,4 +548,57 @@ describe('ReactDOMServerHydration', () => {
     expect(ref.current).toBe(div);
     expect(element.innerHTML).toBe('<div>Hello World</div>');
   });
+
+  // regression test for https://github.com/facebook/react/issues/17170
+  it('should not warn if dangerouslySetInnerHtml=undefined', () => {
+    const domElement = document.createElement('div');
+    const reactElement = (
+      <div dangerouslySetInnerHTML={undefined}>
+        <p>Hello, World!</p>
+      </div>
+    );
+    const markup = ReactDOMServer.renderToStaticMarkup(reactElement);
+    domElement.innerHTML = markup;
+
+    ReactDOM.hydrate(reactElement, domElement);
+
+    expect(domElement.innerHTML).toEqual(markup);
+  });
+
+  it('should warn if innerHTML mismatches with dangerouslySetInnerHTML=undefined and children on the client', () => {
+    const domElement = document.createElement('div');
+    const markup = ReactDOMServer.renderToStaticMarkup(
+      <div dangerouslySetInnerHTML={{__html: '<p>server</p>'}} />,
+    );
+    domElement.innerHTML = markup;
+
+    expect(() => {
+      ReactDOM.hydrate(
+        <div dangerouslySetInnerHTML={undefined}>
+          <p>client</p>
+        </div>,
+        domElement,
+      );
+
+      expect(domElement.innerHTML).not.toEqual(markup);
+    }).toErrorDev(
+      'Warning: Text content did not match. Server: "server" Client: "client"',
+    );
+  });
+
+  it('should warn if innerHTML mismatches with dangerouslySetInnerHTML=undefined on the client', () => {
+    const domElement = document.createElement('div');
+    const markup = ReactDOMServer.renderToStaticMarkup(
+      <div dangerouslySetInnerHTML={{__html: '<p>server</p>'}} />,
+    );
+    domElement.innerHTML = markup;
+
+    expect(() => {
+      ReactDOM.hydrate(<div dangerouslySetInnerHTML={undefined} />, domElement);
+
+      expect(domElement.innerHTML).not.toEqual(markup);
+    }).toErrorDev(
+      'Warning: Did not expect server HTML to contain a <p> in <div>',
+    );
+  });
 });

packages/react-dom/src/client/ReactDOMComponent.js

@@ -1110,12 +1110,11 @@ export function diffHydratedProperties(
       } else if (propKey === DANGEROUSLY_SET_INNER_HTML) {
         const serverHTML = domElement.innerHTML;
         const nextHtml = nextProp ? nextProp[HTML] : undefined;
-        const expectedHTML = normalizeHTML(
-          domElement,
-          nextHtml != null ? nextHtml : '',
-        );
-        if (expectedHTML !== serverHTML) {
-          warnForPropDifference(propKey, serverHTML, expectedHTML);
+        if (nextHtml != null) {
+          const expectedHTML = normalizeHTML(domElement, nextHtml);
+          if (expectedHTML !== serverHTML) {
+            warnForPropDifference(propKey, serverHTML, expectedHTML);
+          }
         }
       } else if (propKey === STYLE) {
         // $FlowFixMe - Should be inferred as not undefined.