**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.
var toggleActivatedState = this._toggleActivatedState;
        return !this.state.activated ? (
          <a ref="x" onClick={toggleActivatedState} />
        ) : (
          <b ref="x" onClick={toggleActivatedState} />
        );
      }
    };
    /**
     * We'll use this to ensure that an old version is not cached when it is
     * reallocated again.
     */
    ChildUpdates = class extends React.Component {
      getAnchor = () => {
        return this.refs.anch;
      };
      render() {
        var className = this.props.anchorClassOn ? 'anchorClass' : '';
        return this.props.renderAnchor ? (
          <a ref="anch" className={className} />
        ) : (
          <b />
        );
      }
    };
  });
  it('should support module pattern components', () => {
    function Child({test}) {