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

[Jan Kokemüller via llvm-commits]

================
@@ -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;
----------------
jiixyj wrote:


> What can certainly be removed from `__repr` and union constructors are the `std::__expected_construct_in_place_from_invoke_tag`/`std::__expected_construct_unexpected_from_invoke_tag` overloads. What do you think?

Well, that didn't work: It seems the function really has to be threaded down into the union and only `std::invoke`d down there...

```
# | In file included from /freebsd/data/linux/git-net/llvm-project/libcxx/test/std/utilities/expected/expected.expected/monadic/transform.pass.cpp:22:                                          
# | In file included from /freebsd/data/linux/git-net/llvm-project/build/include/c++/v1/expected:44:                                                                                            
# | /freebsd/data/linux/git-net/llvm-project/build/include/c++/v1/__expected/expected.h:837:11: error: call to deleted constructor of 'NonCopy'                                                 
# |   837 |         : __val_(std::forward<_Args>(__args)...) {}                                                                                                                                 
# |       |           ^      ~~~~~~~~~~~~~~~~~~~~~~~~~~~                                                                                                                                        
# | /freebsd/data/linux/git-net/llvm-project/build/include/c++/v1/__expected/expected.h:906:11: note: in instantiation of function template specialization 'std::expected<NonCopy, int>::__union
_t<NonCopy, int>::__union_t<NonCopy>' requested here                                                                                                                                            
# |   906 |         : __union_(__tag, std::forward<_Args>(__args)...), __has_val_(true) {}
# |       |           ^
# | /freebsd/data/linux/git-net/llvm-project/build/include/c++/v1/__expected/expected.h:173:9: note: in instantiation of function template specialization 'std::expected<NonCopy, int>::__repr::
__repr<NonCopy>' requested here
# |   173 |       : __repr_(in_place, std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
# |       |         ^
# | /freebsd/data/linux/git-net/llvm-project/build/include/c++/v1/__expected/expected.h:703:14: note: in instantiation of function template specialization 'std::expected<NonCopy, int>::expecte
d<(lambda at /freebsd/data/linux/git-net/llvm-project/libcxx/test/std/utilities/expected/expected.expected/monadic/transform.pass.cpp:220:16) &, int &>' requested here
# |   703 |       return expected<_Up, _Err>(__expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f), __repr_.__union_.__val_);
```

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