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.
// 
              break;
            }
          }
        }
        // 
        // 
        // 
        case FunctionalComponent: {
          const Component = returnFiber.type;
          invariant(
            false,
            '%s(...): Nothing was returned from render. This usually means a ' +
              'return statement is missing. Or, to render nothing, ' +
              'return null.',
            Component.displayName || Component.name || 'Component',
          );
        }
      }
    }
    // 
    return deleteRemainingChildren(returnFiber, currentFirstChild);
  }
  return reconcileChildFibers;
}
exports.reconcileChildFibers = ChildReconciler(true, true);
exports.reconcileChildFibersInPlace = ChildReconciler(false, true);
exports.mountChildFibersInPlace = ChildReconciler(false, false);
exports.cloneChildFibers = function(
  current: Fiber | null,