/**
 * Extends a Promise and its resolved value with properties from a source object.
 *
 * This effectively allows to treat a Promise as a regular object and
 * extend it with custom properties, while maintaining its asynchronous behavior.
 * These added properties are then available on the Promise itself as well as on its resolved value.
 *
 * @template T The type of the `Promise` and its resolved value. This should be an object type.
 * @template U The type of the source object.
 *
 * @param promise The original Promise. The Promise's resolved value must have the same interface as the Promise itself.
 * @param source The source object to merge with the Promise and its resolved value.
 *
 * @returns A new Promise with the properties of both the original Promise and the `source` object.
 *          The resolved value of the new Promise also includes the merged properties.
 *
 * @example
 * ```typescript
 * const originalPromise = Promise.resolve({ initial: 'value' }) as { initial: string } & Promise<{ initial: string }>
 * const sourceObject = { added: 'objectProperty' }
 * const extendedPromise = extendPromise(originalPromise, sourceObject)
 *
 * // Access `added` property directly on the Promise:
 * console.log(extendedPromise.added) // 'objectProperty'
 *
 * extendedPromise.then(result => {
 *   // Access added property on the resolved value:
 *   console.log(result.initial) // 'value'
 *   console.log(result.added) // 'objectProperty'
 * })
 * ```
 */
export declare function extendPromise<T extends object, U>(promise: T & Promise<T>, source: U): T & U & Promise<T & U>;