$form = $this->createForm(SnippetType::class, $notAllowedObject); // type mismatch error

 /**
 * @template-implements DataMapperInterface<SnippetDto>
 */
class SnippetType extends AbstractType implements DataMapperInterface
{
 public function mapDataToForms(mixed $viewData, Traversable $forms): void
 {
 if ($viewData === null) {
 return;
 }
 // Does not make sense to do this check. But we still need to do it. Obvious error: 
 // Instanceof between SnippetDto and SnippetDto will always evaluate to true.
 // 🪪 instanceof.alwaysTrue
 
 //if (!$viewData instanceof SnippetDto) {
 // throw new UnexpectedTypeException($viewData, SnippetDto::class);
 //}
 $forms = iterator_to_array($forms);
 // ...
 }
}