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.
if (shouldClone) {
      const clone = createWorkInProgress(fiber, priority);
      clone.index = 0;
      clone.sibling = null;
      return clone;
    } else {
      
      
      
      fiber.pendingWorkPriority = priority;
      fiber.effectTag = NoEffect;
      fiber.index = 0;
      fiber.sibling = null;
      return fiber;
    }
  }
  function placeChild(