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.
}
      } else {
        switch (newChild.$$typeof) {
          case REACT_ELEMENT_TYPE:
            return placeSingleChild(
              reconcileSingleElement(
                returnFiber,
                currentFirstChild,
                newChild,
                priority,
              ),
            );
          case REACT_COROUTINE_TYPE:
            return placeSingleChild(
              reconcileSingleCoroutine(
                returnFiber,
                currentFirstChild,
                newChild,
                priority,
              ),
            );
          case REACT_YIELD_TYPE:
            return placeSingleChild(
              reconcileSingleYield(
                returnFiber,
                currentFirstChild,
                newChild,
                priority,
              ),
            );