Method refine

packages/zod/src/v4/classic/schemas.ts:151–151

(
    check: Ch,
    params?: string | core.$ZodCustomParams
  )

  // refinements
  refine<Ch extends (arg: core.output<this>) => unknown | Promise<unknown>>(
    check: Ch,
    params?: string | core.$ZodCustomParams
  ): Ch extends (arg: any) => arg is infer R ? this & ZodType<R, core.input<this>> : this;