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

[via libcxx-commits]

https://github.com/huixie90 commented:

Thanks for spotting the issue and send the PR. I am still a bit nervous about object overlapping. Since `expected` can expose its data through several members like `operator*`, `operator->`, `value()`, `error()`, etc.... Could  users trigger this UB by other means that we can not stop from. like
```
std::expected<T, U> e = ...;
e->someMemberThatCouldWriteToObjectThatChangesHasValue()
```
or even,
```
std::expected<T, U> e = ...;
std::construct_at( std::address_of(*e), args...);
```

What do you think?

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