**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)
const oldProps = this._instance.props;
    if (oldProps !== props) {
      
      
      if (
        typeof element.type.getDerivedStateFromProps !== 'function' &&
        typeof this._instance.getSnapshotBeforeUpdate !== 'function'
      ) {
        if (typeof this._instance.componentWillReceiveProps === 'function') {
          this._instance.componentWillReceiveProps(props, context);
        }
        if (
          typeof this._instance.UNSAFE_componentWillReceiveProps === 'function'
        ) {
          this._instance.UNSAFE_componentWillReceiveProps(props, context);
        }
      }
    }
    
    let state = this._newState || oldState;
    if (typeof type.getDerivedStateFromProps === 'function') {
      const partialState = type.getDerivedStateFromProps.call(
        null,
        props,
        state,
      );
      if (partialState != null) {
        state = Object.assign({}, state, partialState);
      }
    }