What's your definition of interesting?

I'm half way through writing a typescript to native code translator (via .Net) compiling a large enough subset of current code with a lot of help from GPT5 and Codex CLI. It has completely blown me away.

I'd like to give you a concrete example which stood out (from by now, dozens). I wanted d.ts files from the .Net Standard Libs. One immediately obvious problem is that .Net would allow classes/interfaces to be redefined if the generic type arity is different. For example, there can be SomeClass<int> and SomeClass<int, int> which are completely separate. TypeScript of course, wouldn't allow this - you can have one with all types defined, but it'd obviously be a mess.

I was stuck with (quite ugly): const users = new List_1<User>(...); instead of const users = new List<User>(...);

So GPT comes up with this:

  declare const __unspecified: unique symbol;
  type __ = typeof __unspecified;

  // Your arity-anchored delegates exist elsewhere:
  //   import("internal/System").Action_0
  //   import("internal/System").Action_1<T1>
  //   import("internal/System").Action_2<T1, T2>
  //   import("internal/System").Action_3<T1, T2, T3>
  //   ... up to 17

  export type Action<
    T1 = __, T2 = __, T3 = __, // ... continue through T17 = __
  > =
    [T1] extends [__] ? import("internal/System").Action_0 :
    [T2] extends [__] ? import("internal/System").Action_1<T1> :
    [T3] extends [__] ? import("internal/System").Action_2<T1, T2> :
    /* next lines follow the same pattern … */
    import("internal/System").Action_3<T1, T2, T3>;

This lets me write:

  const a: Action<number> = (n) => {};        // OK (void)
  const f: Func<number, string> = (s) => 20;  // OK (string -> number)

A human could come up with this, of course. But doing this at scale (there are many such problems which crop up), would take a lot of effort. Btw I'm using Claude for the grunt work (because its faster), but GPT5 is doing all the architecture/thinking/planning/review.