P0722R3: Implement library support for destroying delete

Summary:
This provides the `std::destroying_delete_t` declaration in C++2a and after. (Even when the compiler doesn't support the language feature).

However, the feature test macro `__cpp_lib_destroying_delete` is only defined when we have both language support and  C++2a.


Reviewers: ldionne, ckennelly, serge-sans-paille, EricWF

Reviewed By: EricWF

Subscribers: dexonsmith, riccibruno, christof, jwakely, jdoerfert, mclow.lists, ldionne, libcxx-commits

Differential Revision: https://reviews.llvm.org/D55840

llvm-svn: 361572

1 files changed, 15 insertions, 0 deletions

diff --git a/libcxx/include/new b/libcxx/include/new
index 4cf4c4c1e7b..85e4c4b3fcf 100644
--- a/libcxx/include/new
+++ b/libcxx/include/new
@@ -33,6 +33,12 @@ public:
 };
 
 enum class align_val_t : size_t {}; // C++17
+
+struct destroying_delete_t { // C++20
+  explicit destroying_delete_t() = default;
+};
+inline constexpr destroying_delete_t destroying_delete{}; // C++20
+
 struct nothrow_t {};
 extern const nothrow_t nothrow;
 typedef void (*new_handler)();
@@ -158,6 +164,15 @@ enum align_val_t { __zero = 0, __max = (size_t)-1 };
 #endif
 #endif
 
+#if _LIBCPP_STD_VER > 17
+// Enable the declaration even if the compiler doesn't support the language
+// feature.
+struct destroying_delete_t {
+  explicit destroying_delete_t() = default;
+};
+_LIBCPP_INLINE_VAR constexpr destroying_delete_t destroying_delete{};
+#endif // _LIBCPP_STD_VER > 17
+
 }  // std
 
 #if defined(_LIBCPP_CXX03_LANG)