import { HasTag, SomeSpec, Spec, SpecKind, SpecSuccess, SpecTag, SpectypesError } from './types.js';
/**
 * Creates an optional object property validator spec. Can be used only inside 'object' and 'struct'
 * arguments. Will not produce any validation errors if property equals undefined or is not present
 * in the validated object.
 *
 * @param spec Non-optional spec
 */
export declare const optional: <ItemSpec extends Spec = SomeSpec>(spec: HasTag<ItemSpec, 'optional'> extends true ? SpectypesError<'optional', 'optional'> : HasTag<ItemSpec, 'filter'> extends true ? SpectypesError<'filter', 'optional'> : HasTag<ItemSpec, 'lazy'> extends true ? SpectypesError<'lazy', 'optional'> : ItemSpec) => Spec<[
    'optional',
    ...SpecTag<ItemSpec>
], SpecKind<ItemSpec>, SpecSuccess<ItemSpec> | undefined>;