**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)
render() {
        renderCounter += 1;
        return <div />;
      }
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(<SimpleComponent />);
    expect(renderCounter).toEqual(1);
    const instance = shallowRenderer.getMountedInstance();
    instance.forceUpdate();
    expect(renderCounter).toEqual(2);
  });
  it('should shallow render a function component', () => {
    function SomeComponent(props, context) {
      return (
        <div>
          <div>{props.foo}</div>
          <div>{context.bar}</div>
          <span className="child1" />
          <span className="child2" />
        </div>
      );