Add test for mocked render functions

Treat them as if they return null

Author: acdlite

scripts/fiber/tests-passing.txt

@@ -549,6 +549,7 @@ src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js
 * should not crash encountering low-priority tree
 * throws if non-element passed to top-level render
 * throws if something other than false, null, or an element is returned from render
+* treats mocked render functions as if they return null
 
 src/renderers/dom/shared/__tests__/CSSProperty-test.js
 * should generate browser prefixes for its `isUnitlessNumber`

src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js

@@ -1048,4 +1048,11 @@ describe('disableNewFiberFeatures', () => {
     expect(() => ReactDOM.render(<Render>{999}</Render>, container)).toThrow(/You may have returned undefined/);
     expect(() => ReactDOM.render(<Render>[<div />]</Render>, container)).toThrow(/You may have returned undefined/);
   });
+
+  it('treats mocked render functions as if they return null', () => {
+    class Mocked extends React.Component {}
+    Mocked.prototype.render = jest.fn();
+    ReactDOM.render(<Mocked />, container);
+    expect(container.textContent).toEqual('');
+  });
 });

src/renderers/shared/fiber/ReactChildFiber.js

@@ -1130,7 +1130,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
         case ClassComponent: {
           if (__DEV__) {
             const instance = returnFiber.stateNode;
-            if (instance.render._isMockFunction) {
+            if (instance.render._isMockFunction && typeof newChild === 'undefined') {
               // We allow auto-mocks to proceed as if they're
               // returning null.
               break;
@@ -1254,7 +1254,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
         case ClassComponent: {
           if (__DEV__) {
             const instance = returnFiber.stateNode;
-            if (instance.render._isMockFunction) {
+            if (instance.render._isMockFunction && typeof newChild === 'undefined') {
               // We allow auto-mocks to proceed as if they're returning null.
               break;
             }