オーバーロード無しでfp-ts風のpipe関数を実装

pipe.ts

type Fn<T, U> = (_: T) => U;
type TopFn = Fn<never, unknown>;
type TopFns = ReadonlyArray<TopFn>;

type AssertPipeFns<Arg, Fns extends TopFns> = Fns extends [] ? true
  : Fns extends [
    infer HeadFn extends TopFn,
    ...infer RestFn extends TopFns,
  ] ? HeadFn extends Fn<Arg, infer NextArg> ? AssertPipeFns<NextArg, RestFn>
    : false
  : false;

interface THIS_FUNCION_IS_INVALID_TYPE extends Fn<unknown, never> {}

type FindInvalidFn<Arg, Fns extends TopFns, Return extends TopFns = []> =
  Fns extends [] ? Return
    : Fns extends [
      infer HeadFn extends TopFn,
      ...infer RestFn extends TopFns,
    ]
      ? HeadFn extends Fn<Arg, infer NextArg>
        ? FindInvalidFn<NextArg, RestFn, [...Return, HeadFn]>
      : FindInvalidFn<Arg, RestFn, [...Return, THIS_FUNCION_IS_INVALID_TYPE]>
    : never;

type PipeReturn<T, Fns extends TopFns> = Fns extends
  [...TopFns, infer LastFn extends TopFn] ? ReturnType<LastFn> : T;

declare function pipe<
  T,
  Fns extends TopFns,
>(
  x: T,
  ...fns: AssertPipeFns<T, Fns> extends true ? Fns : FindInvalidFn<T, Fns>
): PipeReturn<T, Fns>;

const a: boolean = pipe(
  1,
  (x: number) => x * 2,
  (x: number) => x.toString(),
  (x: string) => x === "2",
);

// Type Error
const b: boolean = pipe(
  1,
  (x: number) => x * 2,
  (x: string) => x.toString(),
  (x: string) => x === "2",
);