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

[Louis Dionne via libcxx-commits]

Jan =?utf-8?q?Kokemüller?= <jan.kokemueller at gmail.com>,
Jan =?utf-8?q?Kokemüller?= <jan.kokemueller at gmail.com>,
Jan =?utf-8?q?Kokemüller?= <jan.kokemueller at gmail.com>,
Jan =?utf-8?q?Kokemüller?= <jan.kokemueller at gmail.com>,
Jan =?utf-8?q?Kokemüller?= <jan.kokemueller at gmail.com>,
Jan =?utf-8?q?Kokemüller?= <jan.kokemueller at gmail.com>,
Jan =?utf-8?q?Kokemüller?= <jan.kokemueller at gmail.com>
Message-ID:
In-Reply-To: <llvm/llvm-project/pull/68733/libcxx at github.com>


================
@@ -909,6 +878,14 @@ class expected {
         std::__expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
         : __unex_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
 
+    template <class _Union>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(bool __has_val, _Union&& __other) {
+      if (__has_val)
+        std::construct_at(this, std::in_place, std::forward<_Union>(__other).__val_);
----------------
ldionne wrote:

Wouldn't it be cleaner to use this instead?

```
if (__has_val)
  std::construct_at(&__val_, std::in_place, std::forward<_Union>(__other).__val_);
else
  std::construct_at(&__unex_, std::unexpect, std::forward<_Union>(__other).__unex_);
```

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