message: Use std::distance to clarify index calculation
diff: @@ -66,6 +66,6 @@ void RadioButtonGroup::select(std::size_t index)
  void RadioButtonGroup::select(RadioButtonGroup::RadioButton* button)
  {
  - auto index = button - mRadioButtons.data();
  + auto index = std::distance(mRadioButtons.data(), button);
  select(index);
  }
  