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

[via libcxx-commits]

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@@ -136,14 +134,7 @@ class expected {
     noexcept(is_nothrow_copy_constructible_v<_Tp> && is_nothrow_copy_constructible_v<_Err>) // strengthened
     requires(is_copy_constructible_v<_Tp> && is_copy_constructible_v<_Err> &&
              !(is_trivially_copy_constructible_v<_Tp> && is_trivially_copy_constructible_v<_Err>))
-      : __has_val_(__other.__has_val_) {
-    if (__has_val_) {
-      std::construct_at(std::addressof(__union_.__val_), __other.__union_.__val_);
-    } else {
-      std::construct_at(std::addressof(__union_.__unex_), __other.__union_.__unex_);
-    }
-  }
-
+      : __union_(__union_from_expected(__other)), __has_val_(__other.__has_val_) { }
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philnik777 wrote:

I think this should constructed in-place through RVO, but we should definitely check. The function might have to be rewritten as `if (x) { return ...; } else { return ...; }`, but I'm not sure.

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