[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/llvm-project/pull/69673/libcxx at github.com>


================
@@ -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()
----------------
ldionne wrote:

I would do this instead:

```
// This function returns whether the type `_Second` can be stuffed into the tail padding
// of the `_First` type if both of them are given `[[no_unique_address]]`.
template <class _First, class _Second>
inline constexpr bool __fits_in_tail_padding = [](){
  struct __x {
    _LIBCPP_NO_UNIQUE_ADDRESS _First __first;
    _LIBCPP_NO_UNIQUE_ADDRESS _Second __second;
  };
  return sizeof(__x) == sizeof(_First);
}();
```

This is a bit more general. You would then use it as `__fits_in_tail_padding<_Union, bool>`, which is actually even clearer IMO.

Note that I don't really mind the variable-template vs function distinction, that part is just being cute.

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