This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
static childContextTypes = {
        foo: PropTypes.string,
      };
      getChildContext() {
        return {
          foo: 'bar',
        };
      }
      render() {
        return <Child ref={function() {}} />;
      }
    }
    expect(() => ReactTestUtils.renderIntoDocument(<Parent />)).toErrorDev(
      'Warning: Function components cannot be given refs. ' +
        'Attempts to access this ref will fail. ' +
        'Did you mean to use React.forwardRef()?\n\n' +
        'Check the render method ' +
        'of `Parent`.\n' +
        '    in Child (at **)\n' +
        '    in Parent (at **)',
    );
  });
  it('should provide a null ref', () => {
    function Child() {