/**
 * Property decorator that requires the decorated field to hold a boolean.
 * The rule is consumed by {@link validateInstance} and {@link Validatable}.
 *
 * @param target - The class prototype.
 * @param propertyKey - The decorated field.
 *
 * @example
 * ```typescript
 * class Settings {
 *   @IsBoolean enabled = false;
 * }
 * ```
 */
export declare const IsBoolean: (target: object, propertyKey: string | symbol) => void;