> "the definition of std::unreachable is: invokes undefined behaviour" does not mean that all undefined behaviour is equivalent to calling std::unreachable.
Well, obviously not. The definition of UB is that anything can happen, so by transitive property it means that std::unreachable can trigger any behavior.
That's the whole point of UB, and the point of my post: It's UB! You cannot reason about what will happen! The whole point of the post is "No you CANNOT reason about what will happen". So of course I am making no claims about what will happen, just three interesting observations about what did in fact happen when I happened to try it, to prove this point.
> If you wanted a trap instruction, maybe this compiler reliably generates one from a hardcoded null deref.
No, even in the examples in my post I show that it's not reliable. It's UB!
Hell, we get very different output just between -O1 (https://godbolt.org/z/33e1nGE38) and -O2 (https://godbolt.org/z/3coTqP31W).