**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)
}
  }
  getMountedInstance() {
    return this._instance;
  }
  getRenderOutput() {
    return this._rendered;
  }
  render(element: ReactElement | null, context: null | Object = emptyObject) {
    invariant(
      React.isValidElement(element),
      'ReactShallowRenderer render(): Invalid component element.%s',
      typeof element === 'function'
        ? ' Instead of passing a component class, make sure to instantiate ' +
          'it by passing it to React.createElement.'
        : '',
    );
    element = ((element: any): ReactElement);
    
    invariant(
      typeof element.type !== 'string',
      'ReactShallowRenderer render(): Shallow rendering works only with custom ' +
        'components, not primitives (%s). Instead of calling `.render(el)` and ' +
        'inspecting the rendered output, look at `el.props` directly instead.',
      element.type,
    );
    invariant(
      isForwardRef(element) || typeof element.type === 'function',
      'ReactShallowRenderer render(): Shallow rendering works only with custom ' +
        'components, but the provided element type was `%s`.',