/**
 * Returns an iterable that yields the source items or `undefined` when the source is empty.
 *
 * @typeParam T - Element type produced by the source iterable.
 * @param src - Source iterable whose contents are forwarded to the result when available.
 * @returns A deferred iterable producing the original sequence or a single `undefined` when no elements exist.
 *
 * @example
 * ```ts
 * const values = [..._defaultIfEmpty([1, 2, 3])];
 * console.log(values); // [1, 2, 3]
 * ```
 *
 * or using the curried version:
 * ```ts
 * const values = [...pipeInto([], defaultIfEmpty<number>())];
 * console.log(values); // [undefined]
 * ```
 */
export declare function _defaultIfEmpty<T>(src: Iterable<T>): Iterable<T | undefined>;
/**
 * Curried version of {@link _defaultIfEmpty}.
 */
export declare const defaultIfEmpty: <T>() => (src: Iterable<T>) => Iterable<T | undefined>;