alt Hacker NewsStd move doesn’t move ownership. It simply casts into something that could have its ownership taken. Whether or not that actually happens is impossible to identify statically and the value after ownership is consumed is unspecified - sometimes it’s UB to access the value again, sometimes it’s not.
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May be disown would be more descriptive, but the point is that it's intended for transferring of ownership versus copying data.
After moving a value, it needs to remain in a "valid but unspecified state".
How do you mean accessing a valid object is UB?
It is absolutely knowable statically if ownership will be taken. It's not necessarily very easy to do so, but the decision is 100% up to the compiler, as part of overload resolution and optimization choices (like the NRVO analysis that the article mentions). Since ownership is an inherently static concept, it doesn't even make sense to think about "runtime ownership".
That's quite inaccurate.
It needs to remain destructible, and if the type satisfies things like (move-)assignable/copyable, those still need to work as well.
For boxed types, it's likely to set them into some null state, in which case dereferencing them might be ill-formed, but it's a state that is valid for those types anyway.