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,
    element: ReactElement,
    priority: PriorityLevel,
  ): Fiber {
    const key = element.key;
    let child = currentFirstChild;
    while (child !== null) {
      // 
      // 
      if (child.key === key) {
        if (child.type === element.type) {
          deleteRemainingChildren(returnFiber, child.sibling);
          const existing = useFiber(child, priority);
          existing.ref = coerceRef(child, element);
          existing.pendingProps = element.props;