import { AbstractValidator } from '../types';
import type { InferInput, InferValidation, Validation, ValidationOptions, Validator } from '../types';
/**
 * A `Validator` for _optional_ properties (that is `type | undefined`).
 */
export declare class OptionalValidator<T = any, // the type of the "validation", that is the optional type to validate
I = T, // the _input_ type of the "validation", that is anything acceptable
D = undefined> extends AbstractValidator<D extends undefined ? T | undefined : T, I | undefined> {
    validator: Validator<T>;
    defaultValue: T | undefined;
    constructor(validator: Validator<T>);
    constructor(validator: Validator<T>, defaultValue: D);
    constructor(validator: Validator<T>, defaultValue?: void);
    validate(value: unknown, options?: ValidationOptions): D extends undefined ? T | undefined : T;
}
/**
 * Ensure that the property is marked as _optional_ in the `Schema`.
 *
 * @param validation - A `Validation` to be marked as _optional_.
 */
export declare function optional<V extends Validation>(validation: V): OptionalValidator<InferValidation<V>, InferInput<V>, undefined>;
export declare function optional<V extends Validation, D>(validation: V, defaultValue: D): OptionalValidator<InferValidation<V>, InferInput<V>, D>;