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.
return updateCoroutine(returnFiber, oldFiber, newChild, priority);
          } else {
            return null;
          }
        }
        case REACT_YIELD_TYPE: {
          
          
          
          if (key === null) {
            return updateYield(returnFiber, oldFiber, newChild, priority);
          } else {
            return null;
          }
        }
        case REACT_PORTAL_TYPE: {
          if (newChild.key === key) {
            return updatePortal(returnFiber, oldFiber, newChild, priority);
          } else {
            return null;
          }
        }
      }
      if (isArray(newChild) || getIteratorFn(newChild)) {
        
        
        if (key !== null) {
          return null;
        }
        return updateFragment(returnFiber, oldFiber, newChild, priority);