Which bug does it solve?

Translator which supports only strings, I guess.

Easier would be solve it in translator, this is probably not the only place where is non-string value translated.

if (!is_string($message)) {
  return (string) $message;
}

It creates inconsistency with others usage of translate().

With behavior in Nette 3 i can't use Nette\Utils\Html as label (for example with links to terms and conditions). In Nette 2 I create label with link and push it into Nette\Forms\Controls\Checkbox. It works in Nette 3 too: https://github.com/nette/forms/blob/master/src/Forms/Controls/Checkbox.php#L26 But translator converts \Nette\Utils\Html\ instance into string and escape html tags into entities: https://github.com/nette/forms/blob/master/src/Forms/Controls/BaseControl.php#L279 and https://github.com/nette/utils/blob/master/src/Utils/ITranslator.php#L22

In Nette 2 I don't change Nette\Utils\Html in my translator and it works so far:

	if ($message instanceof Nette\Utils\Html) {
			return $message;
	}

In this case would be better use Nette\Forms\Controls\BaseControl::translate instead of $this->getForm()->getTranslator()->translate() in https://github.com/nette/forms/blob/master/src/Forms/Controls/BaseControl.php#L278-L279, because this method has condition for this use case. What do you think?

I see. Translator must return string, but string is escaped by template engine.

Thanks