[llvm] [libc++] Ensure that `std::expected` has no tail padding (PR #69673)

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Jan =?utf-8?q?Kokemüller?= <jan.kokemueller at gmail.com>,
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================
@@ -941,22 +897,90 @@ 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(std::addressof(__val_), std::forward<_Union>(__other).__val_);
+      else
+        std::construct_at(std::addressof(__unex_), std::forward<_Union>(__other).__unex_);
+    }
+
     _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
       requires(is_trivially_destructible_v<_ValueType> && is_trivially_destructible_v<_ErrorType>)
     = default;
 
-    // the expected's destructor handles this
+    // the __expected_repr's destructor handles this
     _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
       requires(!is_trivially_destructible_v<_ValueType> || !is_trivially_destructible_v<_ErrorType>)
     {}
 
-    _LIBCPP_NO_UNIQUE_ADDRESS __empty_t __empty_;
     _LIBCPP_NO_UNIQUE_ADDRESS _ValueType __val_;
     _LIBCPP_NO_UNIQUE_ADDRESS _ErrorType __unex_;
   };
 
-  _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Tp, _Err> __union_;
-  bool __has_val_;
+  struct __expected_repr {
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __expected_repr() = delete;
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __expected_repr(std::in_place_t __tag, _Args&&... __args)
+        : __union_(__tag, std::forward<_Args>(__args)...), __has_val_(true) {}
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __expected_repr(std::unexpect_t __tag, _Args&&... __args)
+        : __union_(__tag, std::forward<_Args>(__args)...), __has_val_(false) {}
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __expected_repr(
+        std::__expected_construct_in_place_from_invoke_tag __tag, _Args&&... __args)
+        : __union_(__tag, std::forward<_Args>(__args)...), __has_val_(true) {}
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __expected_repr(
+        std::__expected_construct_unexpected_from_invoke_tag __tag, _Args&&... __args)
+        : __union_(__tag, std::forward<_Args>(__args)...), __has_val_(false) {}
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __expected_repr(bool __has_val, _Args&&... __args)
+        : __union_(__has_val, std::forward<_Args>(__args)...), __has_val_(__has_val) {}
+
+    _LIBCPP_HIDE_FROM_ABI constexpr __expected_repr(const __expected_repr&) = delete;
+    _LIBCPP_HIDE_FROM_ABI constexpr __expected_repr(const __expected_repr&)
+      requires(is_copy_constructible_v<_Tp> &&
+               is_copy_constructible_v<_Err> &&
+               is_trivially_copy_constructible_v<_Tp> &&
+               is_trivially_copy_constructible_v<_Err>)
+    = default;
+    _LIBCPP_HIDE_FROM_ABI constexpr __expected_repr(__expected_repr&&) = delete;
+    _LIBCPP_HIDE_FROM_ABI constexpr __expected_repr(__expected_repr&&)
+      requires(is_move_constructible_v<_Tp> && is_move_constructible_v<_Err>
+                && is_trivially_move_constructible_v<_Tp> && is_trivially_move_constructible_v<_Err>)
+    = default;
+
+    _LIBCPP_HIDE_FROM_ABI constexpr __expected_repr& operator=(const __expected_repr&) = delete;
+
+    _LIBCPP_HIDE_FROM_ABI constexpr ~__expected_repr()
+      requires(is_trivially_destructible_v<_Tp> && is_trivially_destructible_v<_Err>)
+    = default;
+
+    _LIBCPP_HIDE_FROM_ABI constexpr ~__expected_repr()
+      requires(!is_trivially_destructible_v<_Tp> || !is_trivially_destructible_v<_Err>)
+    {
+      if (__has_val_) {
+        std::destroy_at(std::addressof(__union_.__val_));
+      } else {
+        std::destroy_at(std::addressof(__union_.__unex_));
+      }
+    }
+
+  private:
+    template <class, class>
+    friend class expected;
+
+    [[no_unique_address]] __union_t<_Tp, _Err> __union_;
+    [[no_unique_address]] bool __has_val_;
----------------
philnik777 wrote:

```suggestion
    _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Tp, _Err> __union_;
    _LIBCPP_NO_UNIQUE_ADDRESS bool __has_val_;
```

https://github.com/llvm/llvm-project/pull/69673