Currently, if you define `shouldComponentUpdate` on `React.PureComponent` nothing bad happens and it acts like `React.Component` https://jsfiddle.net/jekmdfva/

We should probably warn in dev since it defaults the point of `PureComponent`
workInProgress,
        newUpdateQueue,
        newInstance,
        newState,
        newProps,
        priorityLevel,
      );
    }
    if (typeof instance.componentDidMount === 'function') {
      workInProgress.effectTag |= Update;
    }
    return true;
  }
  
  function updateClassInstance(
    current: Fiber,
    workInProgress: Fiber,
    priorityLevel: PriorityLevel,
  ): boolean {
    const instance = workInProgress.stateNode;
    resetInputPointers(workInProgress, instance);
    const oldProps = workInProgress.memoizedProps;
    let newProps = workInProgress.pendingProps;
    if (!newProps) {
      
      
      newProps = oldProps;
      invariant(
        newProps != null,
        'There should always be pending or memoized props. This error is ' +