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.
returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    textContent: string,
    priority: PriorityLevel,
  ): Fiber {
    // 
    // 
    if (currentFirstChild !== null && currentFirstChild.tag === HostText) {
      // 
      // 
      deleteRemainingChildren(returnFiber, currentFirstChild.sibling);
      const existing = useFiber(currentFirstChild, priority);
      existing.pendingProps = textContent;
      existing.return = returnFiber;
      return existing;
    }
    // 
    // 
    deleteRemainingChildren(returnFiber, currentFirstChild);
    const created = createFiberFromText(
      textContent,
      returnFiber.internalContextTag,
      priority,
    );
    created.return = returnFiber;
    return created;
  }
  function reconcileSingleElement(