/**
 * @jorvel/runtime — useOptimistic
 *
 * A React-18-compatible optimistic-UI hook with the React 19 `useOptimistic`
 * shape. Show a predicted state immediately while a mutation is in flight; when
 * the real state arrives (the base `state` prop changes), the optimistic
 * overlay is dropped automatically.
 *
 * On React 19 you can use the built-in `useOptimistic`; this gives the same
 * ergonomics on 18 and degrades to it cleanly.
 */
/**
 * @param state    The authoritative state (e.g. server data).
 * @param updateFn Applies one optimistic action onto the current state.
 * @returns `[optimisticState, addOptimistic]`. Call `addOptimistic(action)`
 *          right before you kick off the async mutation; the overlay clears
 *          when `state` next changes identity (the mutation resolved).
 */
export declare function useOptimistic<State, Action = State>(state: State, updateFn: (currentState: State, optimisticValue: Action) => State): [State, (action: Action) => void];
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