import { IterableX } from '../iterable';
/**
 * Returns the elements of the specified sequence or the type parameter's default value in a singleton collection if the sequence is empty.
 * @param {Iterable<T>} source The sequence to return a default value for if it is empty.
 * @param {T} defaultValue The default value if the sequence is empty.
 * @return An that contains the default value if source is empty; otherwise, source.
 */
export declare function defaultIfEmpty<T>(source: Iterable<T>, defaultValue: T): IterableX<T>;