[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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huixie90 wrote:

 I wonder if this function call introduces extra move/copy of the `__union_t`. What about making this function `__union_from_expected` a constructor of the `__union_t`

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