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.
const key = coroutine.key;
    let child = currentFirstChild;
    while (child !== null) {
      // 
      // 
      if (child.key === key) {
        if (child.tag === CoroutineComponent) {
          deleteRemainingChildren(returnFiber, child.sibling);
          const existing = useFiber(child, priority);
          existing.pendingProps = coroutine;
          existing.return = returnFiber;
          return existing;
        } else {
          deleteRemainingChildren(returnFiber, child);
          break;
        }
      } else {
        deleteChild(returnFiber, child);
      }
      child = child.sibling;
    }
    const created = createFiberFromCoroutine(
      coroutine,
      returnFiber.internalContextTag,
      priority,
    );
    created.return = returnFiber;
    return created;
  }
  function reconcileSingleYield(