https://jsfiddle.net/3apqnhmw/

```jsx
class Foo extends React.Component {
  render() {
     return <div>Foo</div>
  }
}

class App extends React.Component {
  render() {
    return Foo
  }
}
```

When `App` returns `Foo` instead of `<Foo />` accidentally, Fiber does not catch this.
// 
    // 
    // 
    // 
    const disableNewFiberFeatures = ReactFeatureFlags.disableNewFiberFeatures;
    // 
    const isObject = typeof newChild === 'object' && newChild !== null;
    if (isObject) {
      // 
      // 
      if (disableNewFiberFeatures) {
        switch (newChild.$$typeof) {
          case REACT_ELEMENT_TYPE:
            return placeSingleChild(
              reconcileSingleElement(
                returnFiber,
                currentFirstChild,
                newChild,
                priority,
              ),
            );
          case REACT_PORTAL_TYPE:
            return placeSingleChild(
              reconcileSinglePortal(
                returnFiber,
                currentFirstChild,
                newChild,
                priority,
              ),
            );