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

[Jan Kokemüller via libcxx-commits]

================
@@ -88,8 +88,263 @@ _LIBCPP_HIDE_FROM_ABI void __throw_bad_expected_access(_Arg&& __arg) {
 #  endif
 }
 
+struct __expected_invoke_tag {};
+
+template <bool _NoUnique, class _Tp>
+class __expected_conditional_no_unique_address {
+  struct __unique {
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __unique(_Args&&... __args)
+        : __v(std::forward<_Args>(__args)...) {}
+
+    template <class _Func, class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __unique(
+        __expected_invoke_tag, _Func&& __f, _Args&&... __args)
+        : __v(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
+
+    _Tp __v;
+  };
+
+  struct __no_unique {
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __no_unique(_Args&&... __args)
+        : __v(std::forward<_Args>(__args)...) {}
+
+    template <class _Func, class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __no_unique(
+        __expected_invoke_tag, _Func&& __f, _Args&&... __args)
+        : __v(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
+
+    _LIBCPP_NO_UNIQUE_ADDRESS _Tp __v;
+  };
+
+public:
+  using __type = std::conditional<_NoUnique, __no_unique, __unique>::type;
+};
+
+template <class _Union>
+_LIBCPP_HIDE_FROM_ABI constexpr bool __expected_can_stuff_tail()
+{
+  struct __x {
+  private:
+    _LIBCPP_NO_UNIQUE_ADDRESS _Union __union_;
+    _LIBCPP_NO_UNIQUE_ADDRESS bool __has_val_;
+  };
+  return sizeof(__x) == sizeof(_Union);
+}
+
+template <class _Tp, class _Err>
+class __expected_base {
+  // use named union because [[no_unique_address]] cannot be applied to an unnamed union,
+  // also guaranteed elision into a potentially-overlapping subobject is unsettled (and
+  // it's not clear that it's implementable, given that the function is allowed to clobber
+  // the tail padding) - see https://github.com/itanium-cxx-abi/cxx-abi/issues/107.
+  union __union_t {
----------------
jiixyj wrote:

Should I extract the `__repr` out of the `__expected_base` as well?

What I like about the current approach is that both `__union` and `__repr` are tucked away inside the `__expected_base` class and cannot be used anywhere else by accident. Outside, they would have to be templates again and have names with a prefix (like `__expected_union_t`). What do you think?

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