This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
return <div ref="me" />;
    }
    expect(function() {
      ReactTestUtils.renderIntoDocument(<Child test="test" />);
    }).toThrowError(
      __DEV__
        ? 'Function components cannot have string refs. We recommend using useRef() instead.'
        : 
          
          'Element ref was specified as a string (me) but no owner was set. This could happen for one of' +
            ' the following reasons:\n' +
            '1. You may be adding a ref to a function component\n' +
            "2. You may be adding a ref to a component that was not created inside a component's render method\n" +
            '3. You have multiple copies of React loaded\n' +
            'See https://reactjs.org/link/refs-must-have-owner for more information.',
    );
  });
  it('should warn when given a string ref', () => {
    function Indirection(props) {
      return <div>{props.children}</div>;
    }
    class ParentUsingStringRef extends React.Component {