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

[Jan Kokemüller via libcxx-commits]

================
@@ -88,8 +88,353 @@ _LIBCPP_HIDE_FROM_ABI void __throw_bad_expected_access(_Arg&& __arg) {
 #  endif
 }
 
+struct __conditional_no_unique_address_invoke_tag {};
+
+// This class implements an object with `[[no_unique_address]]` conditionally applied to it,
+// based on the value of `_NoUnique`.
+//
+// A member of this class must always have `[[no_unique_address]]` applied to
+// it. Otherwise, the `[[no_unique_address]]` in the "`_NoUnique == true`" case
+// would not have any effect. In the `false` case, the `__v` is not
+// `[[no_unique_address]]`, so nullifies the effects of the "outer"
+// `[[no_unique_address]]` regarding data layout.
+//
+// If we had a language feature, this class would basically be replaced by `[[no_unique_address(condition)]]`.
+template <bool _NoUnique, class _Tp>
+struct __conditional_no_unique_address;
+
+template <class _Tp>
+struct __conditional_no_unique_address<true, _Tp> {
+  template <class... _Args>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __conditional_no_unique_address(in_place_t, _Args&&... __args)
+      : __v(std::forward<_Args>(__args)...) {}
+
+  template <class _Func, class... _Args>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __conditional_no_unique_address(
+      __conditional_no_unique_address_invoke_tag, _Func&& __f, _Args&&... __args)
+      : __v(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
+
+  _LIBCPP_NO_UNIQUE_ADDRESS _Tp __v;
----------------
jiixyj wrote:

I agree, this is confusing. I thought of using `__v` ("value") here for the member name as `__conditional_no_unique_address` is just a wrapper type around some "value", similar to other "wrapper types" like [in_value_result](<https://en.cppreference.com/w/cpp/algorithm/ranges/return_types/in_value_result>). But here is clashes with the `__val_` name of the union...

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