[libcxx-commits] [libcxx] [libc++] Add tombstone traits and use them in optional (PR #98498)

[Louis Dionne via libcxx-commits]

================
@@ -120,7 +123,13 @@ class _LIBCPP_TEMPLATE_VIS reference_wrapper : public __weak_result_type<_Tp> {
 #if _LIBCPP_STD_VER >= 17
 template <class _Tp>
 reference_wrapper(_Tp&) -> reference_wrapper<_Tp>;
-#endif
+
+template <class _Tp>
+struct __tombstone_traits<reference_wrapper<_Tp>> {
+  static constexpr uintptr_t __disengaged_value_  = __builtin_offsetof(reference_wrapper<_Tp>, __f_);
+  static constexpr size_t __is_disengaged_offset_ = 0;
+};
----------------
ldionne wrote:

Most of these traits require a bit of thinking in order to understand what we're taking advantage of. It would be helpful to have a 1-2 line comment for each describing "Here's the invariant of the type and here's the 'unreachable' state of the object we're using to store information".

https://github.com/llvm/llvm-project/pull/98498