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.
workInProgress: Fiber,
): void {
  invariant(
    current === null || workInProgress.child === current.child,
    'Resuming work not yet implemented.',
  );
  if (workInProgress.child === null) {
    return;
  }
  let currentChild = workInProgress.child;
  let newChild = createWorkInProgress(
    currentChild,
    currentChild.pendingWorkPriority,
  );
  // 
  newChild.pendingProps = currentChild.pendingProps;
  workInProgress.child = newChild;
  newChild.return = workInProgress;
  while (currentChild.sibling !== null) {
    currentChild = currentChild.sibling;
    newChild = newChild.sibling = createWorkInProgress(
      currentChild,
      currentChild.pendingWorkPriority,
    );
    newChild.pendingProps = currentChild.pendingProps;
    newChild.return = workInProgress;
  }
  newChild.sibling = null;
};