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<?php declare(strict_types = 1); /** * @template-covariant T */ final class Collection { /** * @param list<T> $items */ public function __construct( private array $items = [], ) { } /** * @template TValue * @param Closure(T, int): TValue $callback * @return Collection<TValue> */ public function map(Closure $callback): self { $mapped = []; foreach ($this->items as $key => $item) { $mapped[] = $callback($item, $key); } return new self($mapped); } } enum X: string { case CASE1 = 'case1'; case CASE2 = 'case2'; } (new Collection(['case1']))->map(X::from(...));