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`
oldState !== current.memoizedState
        ) {
          workInProgress.effectTag |= Update;
        }
      }
      return false;
    }
    const shouldUpdate = checkShouldComponentUpdate(
      workInProgress,
      oldProps,
      newProps,
      oldState,
      newState,
      newContext,
    );
    if (shouldUpdate) {
      if (typeof instance.componentWillUpdate === 'function') {
        if (__DEV__) {
          startPhaseTimer(workInProgress, 'componentWillUpdate');
        }
        instance.componentWillUpdate(newProps, newState, newContext);
        if (__DEV__) {
          stopPhaseTimer();
        }
      }
      if (typeof instance.componentDidUpdate === 'function') {
        workInProgress.effectTag |= Update;
      }
    } else {