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

[A. Jiang via llvm-commits]

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@@ -955,8 +956,17 @@ class expected {
     _LIBCPP_NO_UNIQUE_ADDRESS _ErrorType __unex_;
   };
 
+  _LIBCPP_HIDE_FROM_ABI static constexpr auto __calculate_padding() {
+    struct __calc_expected {
+      _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Tp, _Err> __union_;
+      bool __has_val_;
+    };
+    return sizeof(__calc_expected) - __libcpp_datasizeof<__calc_expected>::value;
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frederick-vs-ja wrote:

It seems we can determine the size of the padding via only `sizeof(__union_t<_Tp, _Err>)` and `__libcpp_datasizeof<__union_t<_Tp, _Err>>::value` without inventing a new type.

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