**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)
componentWillReceivePropsParams.push(...args);
        this.setState((...innerArgs) => {
          setStateParams.push(...innerArgs);
          return updatedState;
        });
      }
      UNSAFE_componentWillUpdate(...args) {
        componentWillUpdateParams.push(...args);
      }
      shouldComponentUpdate(...args) {
        shouldComponentUpdateParams.push(...args);
        return true;
      }
      render() {
        return null;
      }
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(
      React.createElement(SimpleComponent, initialProp),
      initialContext,
    );
    expect(componentDidUpdateParams).toEqual([]);
    expect(componentWillReceivePropsParams).toEqual([]);
    expect(componentWillUpdateParams).toEqual([]);
    expect(setStateParams).toEqual([]);
    expect(shouldComponentUpdateParams).toEqual([]);
    
    shallowRenderer.render(