def METHOD_NAME(capfd, use_varlib):
    """
    Test extrapolating, with default (MutatorMath) and with varlib.
    Using varlib should fail (output should not be extrapolated) because
    extrapolation is not supported by varlib.
    """
    runner_args = ['-t', TOOL, '-o', 'v', 'd', f'_{get_input_path("extrapolation.designspace")}']  # noqa: E501
    if use_varlib:
        runner_args.append('=use-varlib')
    runner(runner_args)
    captured = capfd.readouterr()
    tool = "fontTools.varlib" if use_varlib else "MutatorMath"
    assert f"Building 2 instances with {tool} ..." in captured.err
    for ufo_filename in ("Dummy-ExtraPlus.ufo", "Dummy-ExtraMinus.ufo"):
        expected_path = _get_output_path(ufo_filename, 'expected_output')
        actual_path = _get_output_path(ufo_filename, 'temp_output')
        if use_varlib:
            assert not differ([expected_path, actual_path])
            assert "Extrapolation is not supported with varlib (Dummy Extra Plus weight: 1500.0)" in captured.err  # noqa: E501
        else:
            assert differ([expected_path, actual_path])