This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
render() {
        return React.createElement(FunctionComponent, {
          name: 'A',
          ref: () => {},
        });
      }
    }
    let instance3;
    expect(() => {
      instance3 = ReactTestUtils.renderIntoDocument(<NamedParentNotUsingJSX />);
    }).toErrorDev('Warning: Function components cannot be given refs.');
    
    instance3.forceUpdate();
    
    ReactTestUtils.renderIntoDocument(<NamedParentNotUsingJSX />);
  });
  
  
  it('should warn when giving a function ref with context', () => {
    function Child() {
      return null;
    }
    Child.contextTypes = {
      foo: PropTypes.string,
    };
    class Parent extends React.Component {