Wrapped-signature-experiment.ts

import { HasKeys, IsEmpty, RequiredFields } from "."

export type OptionalIfEmpty<T> = [
  ...(IsEmpty<T> extends true
    ? [never?]
    : HasKeys<RequiredFields<T>> extends true
    ? [T]
    : [T?])
]

/**
 * Wrap function signature to conditionally disallow
 * last arg if its an empty object/interface OR allow
 * omitting it if all fields are optional
 *
 * Doesn't properly infer Generics from wrapped function ?? :((
 */
export type FuncWithOptionalLastArg<Fn extends Function> = Fn extends (
  ...args: [...infer Params, infer LastParam]
) => infer Return
  ? (...args: [...Params, ...OptionalIfEmpty<LastParam>]) => Return
  : never