**Do you want to request a *feature* or report a *bug*?**
report a bug

**What is the current behavior?**
When I call setState with a callback in a test using react shallow renderer (via enzyme), the callback gets called and `this.state` is still the old state.

EDIT: This seems limited to componentWillMount

**If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem via https://jsfiddle.net or similar (template for React 16: https://jsfiddle.net/Luktwrdm/, template for React 15: https://jsfiddle.net/hmbg7e9w/).**

EDIT: Reproduce with this https://github.com/bdwain/setstate-callback-bug

**What is the expected behavior?**
When the setState callback gets called, it should have access to the new state.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
Version 16.
It worked in v15 with enzyme 2.

EDIT: I think this is because in componentWillMount, [this line](https://github.com/facebook/react/blob/master/packages/react-test-renderer/src/ReactShallowRenderer.js#L67) causes the render method in the shallow wrapper to return early, before it updates the state.
render() {
        return (
          <div>
            <Child />
          </div>
        );
      }
    }
    spyOnDev(console, 'warn');
    var markup = ReactDOMServer.renderToString(<Parent />);
    
    var container = document.createElement('div');
    container.innerHTML = markup;
    ReactDOM.render(<Parent />, container);
    if (__DEV__) {
      expect(console.warn.calls.count()).toBe(1);
      expect(console.warn.calls.argsFor(0)[0]).toContain(
        'render(): Calling ReactDOM.render() to hydrate server-rendered markup ' +
          'will stop working in React v17. Replace the ReactDOM.render() call ' +
          'with ReactDOM.hydrate() if you want React to attach to the server HTML.',
      );
      console.warn.calls.reset();
    }
    
    container = document.createElement('div');
    container.innerHTML = markup;
    ReactDOM.hydrate(<Parent />, container);
    if (__DEV__) {
      expect(console.warn.calls.count()).toBe(0);
    }