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

[Louis Dionne via libcxx-commits]

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In-Reply-To: <llvm.org/llvm/llvm-project/pull/69673 at github.com>


================
@@ -88,8 +88,289 @@ _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(_Args&&... __args)
----------------
ldionne wrote:

I would suggest using `in_place_t` as a tag here to completely remove the possibility that this constructor is going to be used in the case where we do:

```
__conditional_no_unique_address<Bool, T> bar(...);
__conditional_no_unique_address<Bool, T> foo(bar); // what does this call?
```

So instead I would use

```
constexpr explicit __conditional_no_unique_address(in_place_t, _Args&&... __args)
```

and the same below

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