[libcxx-commits] [libcxx] [libc++] Fix UB in <expected> related to "has value" flag (#68552) (PR #68733)

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philnik777 wrote:

> Would you folks agree with the following general guideline?
> 
> > Never use `std::construct_at` on something that is marked as `[[no_unique_address]]` because it potentially leads to overwriting tail padding that you might not own (in case the compiler jammed another object into it).
> 
> @zygoloid Is that basically the mental model we should have? If so, would it make sense to have a compiler warning for that?

I think so. Basically, if we play around with object lifetime in a non-trivial way, we shouldn't use `[[no_unique_address]]` on the object that we're playing around with.

https://github.com/llvm/llvm-project/pull/68733