[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 {};
----------------
jiixyj wrote:

The constructor with this tag is needed if the type `_Tp` inside the `__conditional_no_unique_address` is neither copyable nor movable. In this case one has to rely on guaranteed copy elision to maneuver the object into place. For example, the `__union_t` type often is neither copyable nor movable and you have to do this dance to construct it:

```c++
        : __union_(__conditional_no_unique_address_invoke_tag{},
                   [&] { return __make_union(__has_val, std::forward<_OtherUnion>(__other)); }),
```

...where previously, without `__conditional_no_unique_address`, one could simply do this:

```c++
        : __union_(__make_union(__has_val, std::forward<_OtherUnion>(__other))),
```

I added a comment to the tag type.

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