**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.
it('throws usefully when rendering badly-typed elements', () => {
    spyOnDev(console, 'error');
    const shallowRenderer = createRenderer();
    var Undef = undefined;
    expect(() => shallowRenderer.render(<Undef />)).toThrowError(
      'ReactShallowRenderer render(): Shallow rendering works only with custom ' +
        'components, but the provided element type was `undefined`.',
    );
    var Null = null;
    expect(() => shallowRenderer.render(<Null />)).toThrowError(
      'ReactShallowRenderer render(): Shallow rendering works only with custom ' +
        'components, but the provided element type was `null`.',
    );
    var Arr = [];
    expect(() => shallowRenderer.render(<Arr />)).toThrowError(
      'ReactShallowRenderer render(): Shallow rendering works only with custom ' +
        'components, but the provided element type was `array`.',
    );
    var Obj = {};
    expect(() => shallowRenderer.render(<Obj />)).toThrowError(
      'ReactShallowRenderer render(): Shallow rendering works only with custom ' +
        'components, but the provided element type was `object`.',
    );
    if (__DEV__) {
      
      expect(console.error.calls.count()).toBe(4);
    }
  });
});