Ternary chains are pretty common in TS, since it’s the main control flow. Are you a comfortable TS user normally?
Are you saying ternary chains using sentinels for arity inference is pretty common? I would disagree.
> since it’s the main control flow
Perhaps you're saying ternery chains are common in TS code? That's a very different thing though - the code above is not for runtime behavior.
Why would Range need a sentinel?
My point is that using a sentinel to bridge TypeScript's lack of generic arity-based specialization is a non-trivial problem. After you mentioned it, I looked for examples on Google and couldn't find anything that matches precisely.
I'm not claiming humans can't solve this, or that gpt5 invented something fundamentally new. My original point was about productivity at scale. Having a model apply the right solution across dozens of similar problems, rather than me manually figuring out each one.