You basically don't want `observe` to be a conforming Hardened Implementation. I'd be fine with that. I don't remember why the wording ended up allowing it, but I think it would be fine to remove `observe` from the set of allowable semantics for a Hardened Implementation. I don't think that contradicts the design intent. In a way, that's mostly a "marketing" distinction: In practice, that means you can still select `observe` or `ignore`, but that won't qualify as a Hardened Implementation. I think that achieves the best combination of usefulness and a formal guarantee of termination under "Hardened" mode.
But, just to be clear, many large adopters will need the ability to select the observe semantic in order to deploy this at a large scale. That's an extremely useful tool to have. I don't care if that's not called a "Hardened Implementation", but it should be possible.
The second concern is about how standard library code can be written to produce a hardened implementation. In the C++26 timeframe, since Contracts are a MVP, producing a hardened implementation will basically require the user to select a set of appropriate evaluation semantics (e.g. quick_enforce or enforce) throughout all TUs that use the Standard Library, or the compiler to implement a tagging extension on top of the Contracts MVP. Once we have a standardized tagging mechanism, we'll be able to expand the set of valid Hardened programs to also include (for example) the standard library using `quick_enforce` but the user code using `observe`, and to do so in a way that is portable. In practice, I expect this to mean that in order to get an official "Hardened Implementation" in C++26 you'll want to use terminating semantics throughout your whole program. That doesn't make Contracts any less useful on their own or in combination with hardening, it just means that you'll need to abide by stricter rules than perhaps necessary if you want the official "Hardening Implementation" stamp, until we have a tagging mechanism.
TLDR: I think the first wording suggestion in your paper makes sense. That makes only `enforce` and `quick_enforce` be valid evaluation semantics for Hardened Implementations and removes `observe`. Contracts and hardening are still useful with the Contracts MVP, and they'll be more useful once we have additional Contracts features like tagging. That's not a reason to kill either.
Cheers,
Louis