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`
const fiber = ReactInstanceMap.get(instance);
      const priorityLevel = getPriorityContext(fiber, false);
      callback = callback === undefined ? null : callback;
      if (__DEV__) {
        warnOnInvalidCallback(callback, 'forceUpdate');
      }
      addForceUpdate(fiber, callback, priorityLevel);
      scheduleUpdate(fiber, priorityLevel);
    },
  };
  function checkShouldComponentUpdate(
    workInProgress,
    oldProps,
    newProps,
    oldState,
    newState,
    newContext,
  ) {
    if (
      oldProps === null ||
      (workInProgress.updateQueue !== null &&
        workInProgress.updateQueue.hasForceUpdate)
    ) {
      
      return true;
    }
    const instance = workInProgress.stateNode;
    if (typeof instance.shouldComponentUpdate === 'function') {