This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
render() {
        return (
          <Indirection>
            <FunctionComponent
              name="A"
              ref={arg => {
                expect(arg).toBe(null);
              }}
            />
          </Indirection>
        );
      }
    }
    expect(() =>
      ReactTestUtils.renderIntoDocument(<ParentUsingFunctionRef />),
    ).toErrorDev(
      'Warning: Function components cannot be given refs. ' +
        'Attempts to access this ref will fail. ' +
        'Did you mean to use React.forwardRef()?\n\n' +