import { type Observable } from "rxjs";
/**
 * Per-field observable derivation factory. Enforces the §19.4 invariant:
 *
 * - `distinctUntilChanged` — prevents re-emission storms when the upstream
 *   state object ticks but the field a consumer cares about hasn't changed.
 *   Without this, every keystroke would re-render every async-pipe binding
 *   in the consumer's template even when nothing changed for that binding.
 * - `shareReplay({ bufferSize: 1, refCount: true })` — guarantees that
 *   multiple subscribers (multiple `async` pipes in a template, e.g.
 *   `*ngIf="isLoading$ | async"` and `[disabled]="isLoading$ | async"`)
 *   share a single upstream subscription. `refCount: true` is non-obvious
 *   but essential: without it, the inner subscription survives after the
 *   last subscriber unmounts, leaking the chain back to `state$`.
 *
 * Every public observable on {@link AIAutocompleteController} must go through
 * this factory — no exceptions.
 *
 * @internal
 */
export declare function deriveField<T, R>(source$: Observable<T>, selector: (value: T) => R, equalityFn?: (a: R, b: R) => boolean): Observable<R>;
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