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

[Jan Kokemüller via libcxx-commits]

jiixyj wrote:

> To fix that we'd have to explicitly pad out the object.

+1. Even `const` objects are not safe:

```c++
struct bool_with_padding {
    alignas(8) bool b = false;
};

struct s : std::expected<bool_with_padding, bool> {
    const bool please_dont_overwrite_me[6] =  //
        {true, true, true, true, true, true};
};

static_assert(sizeof(s) == sizeof(bool_with_padding));

int main() {
    s s;

    s.emplace();

    assert(s.please_dont_overwrite_me[5]); // will abort
    assert(s.please_dont_overwrite_me[4]);
    assert(s.please_dont_overwrite_me[3]);
    assert(s.please_dont_overwrite_me[2]);
    assert(s.please_dont_overwrite_me[1]);
    assert(s.please_dont_overwrite_me[0]);
}
```

https://godbolt.org/z/TPoEqz7Wc

That doesn't look right :/

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