**Do you want to request a *feature* or report a *bug*?**
bug

**What is the current behavior?**
In normal rendering, you can memoize a class-based component: https://jsfiddle.net/586ea3cx/

With the shallow renderer, it seems like you can't: https://jsfiddle.net/odj217Lv/1/

This might be blocking https://github.com/airbnb/enzyme/pull/1914, in which tests are failing with `Cannot call a class as a function`. (it's tough to repro stuff with the shallow renderer)
let shouldUpdate = true;
    if (this._forcedUpdate) {
      shouldUpdate = true;
      this._forcedUpdate = false;
    } else if (typeof this._instance.shouldComponentUpdate === 'function') {
      shouldUpdate = !!this._instance.shouldComponentUpdate(
        props,
        state,
        context,
      );
    } else if (type.prototype && type.prototype.isPureReactComponent) {
      shouldUpdate =
        !shallowEqual(oldProps, props) || !shallowEqual(oldState, state);
    }
    if (shouldUpdate) {
      
      
      if (
        typeof element.type.getDerivedStateFromProps !== 'function' &&
        typeof this._instance.getSnapshotBeforeUpdate !== 'function'
      ) {
        if (typeof this._instance.componentWillUpdate === 'function') {
          this._instance.componentWillUpdate(props, state, context);
        }
        if (typeof this._instance.UNSAFE_componentWillUpdate === 'function') {
          this._instance.UNSAFE_componentWillUpdate(props, state, context);
        }
      }
    }
    this._instance.context = context;