/**
 * Wraps an async function so that successive calls with identical arguments
 * are served from an in-memory cache instead of re-executing the function.
 * An optional TTL causes cache entries to expire after a given number of
 * milliseconds, and a custom `keyFn` lets callers control how arguments
 * are serialized into cache keys.
 *
 * @param fn - The async function to memoize.
 * @param options - Memoization options.
 * @param options.ttl - Time-to-live in milliseconds. After this duration the
 * cached entry is evicted and `fn` is called again. Omit for indefinite caching.
 * @param options.keyFn - Custom function to derive a string cache key from the
 * call arguments. Defaults to `JSON.stringify(args)`.
 * @returns A new function with the same signature as `fn` that transparently
 * caches resolved values.
 *
 * @throws {Error} If `options.ttl` is a negative number.
 * @example
 * // Basic memoization — second call is served from cache
 * const fetchUser = asyncMemoize(async (id: number) => {
 *   const res = await fetch(`/api/users/${id}`);
 *   return res.json();
 * });
 * await fetchUser(1); // hits the network
 * await fetchUser(1); // served from cache
 *
 * @example
 * // With a 5-second TTL
 * const getConfig = asyncMemoize(
 *   async () => fetchRemoteConfig(),
 *   { ttl: 5000 },
 * );
 *
 * @example
 * // Custom key function for complex arguments
 * const search = asyncMemoize(
 *   async (query: string, page: number) => apiSearch(query, page),
 *   { keyFn: (query, page) => `${query}:${page}` },
 * );
 *
 * @note Rejected promises are NOT cached. A failed call will be retried on the
 * next invocation with the same arguments. Create a new memoized function
 * instance to reset the cache entirely.
 *
 * @complexity Time: O(1) per cached call, Space: O(k) where k is unique key count
 */
export declare function asyncMemoize<T, Args extends unknown[]>(fn: (...args: Args) => Promise<T>, options?: {
    ttl?: number;
    keyFn?: (...args: Args) => string;
}): (...args: Args) => Promise<T>;
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