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

[Louis Dionne via libcxx-commits]

================
@@ -112,8 +113,17 @@ private:
   friend bool has_facet(const locale&) _NOEXCEPT;
   template <class _Facet>
   friend const _Facet& use_facet(const locale&);
+
+  friend struct __tombstone_traits<locale>;
 };
 
+#if _LIBCPP_STD_VER >= 17
+template <>
+struct __tombstone_traits<locale> : __tombstone_traits_assume_aligned_pointer {
+  static_assert(__builtin_offsetof(locale, __locale_) == 0);
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ldionne wrote:

Just to be safe, we might also want to `static_assert` that the size of `std::locale` is exactly the size of a pointer. While the code would still work without that assumption, I think we'd want to be notified if we change the layout of `locale` (e.g. under some ABI config) and revisit whether these `tombstone_traits` still make sense as-is.

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