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

[Jan Kokemüller via libcxx-commits]

================
@@ -893,11 +854,15 @@ class expected {
   }
 
 private:
-  struct __empty_t {};
----------------
jiixyj wrote:

And previously, this was needed when the `expected` was initialized like this (pseudo code):

```c++
expected(const expected& other) : union_(), has_value_(other.has_value_) {
  // `std::construct_at` into `union_`
}
```

...but now, the union is never value-initialized any longer. All `expected` constructors will either initialize value or error, and make use of the member initializer list:

```c++
expected(const expected& other) : union_(other.has_value_, other.union_), has_value_(other.has_value_) { }
```

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