**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)
return {
          count: prevState.count + props.incrementBy,
          other: 'foobar',
        };
      }
      render() {
        return (
          <div>{`count:${this.state.count}, other:${this.state.other}`}</div>
        );
      }
    }
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<SimpleComponent incrementBy={2} />);
    expect(result).toEqual(<div>count:3, other:foobar</div>);
  });
  it('can setState in componentWillMount when shallow rendering', () => {
    class SimpleComponent extends React.Component {
      UNSAFE_componentWillMount() {
        this.setState({groovy: 'doovy'});
      }
      render() {
        return <div>{this.state.groovy}</div>;
      }
    }
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<SimpleComponent />);
    expect(result).toEqual(<div>doovy</div>);
  });
  it('can setState in componentWillMount repeatedly when shallow rendering', () => {
    class SimpleComponent extends React.Component {