[libcxx-commits] [clang] [libcxx] [Clang][libc++] Implement C++23 std::start_lifetime_as (P2590R2, P2679R2) (PR #196286)

[A. Jiang via libcxx-commits]

================
@@ -0,0 +1,76 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// <memory>
+// template<class T> T* start_lifetime_as(void* p) noexcept;
+// template<class T> const T* start_lifetime_as(const void* p) noexcept;
+// template<class T> volatile T* start_lifetime_as(volatile void* p) noexcept;
+// template<class T> const volatile T* start_lifetime_as(const volatile void* p) noexcept;
+
+#include <memory>
+#include <cassert>
+#include <type_traits>
+
+struct Trivial {
+  int x;
+  float y;
+};
+
+struct NestedArray {
+  int arr[3][4];
+};
+
+static_assert(noexcept(std::start_lifetime_as<int>(std::declval<void*>())));
+static_assert(noexcept(std::start_lifetime_as<int>(std::declval<const void*>())));
+static_assert(noexcept(std::start_lifetime_as<int>(std::declval<volatile void*>())));
+static_assert(noexcept(std::start_lifetime_as<int>(std::declval<const volatile void*>())));
+
+static_assert(std::is_same_v<decltype(std::start_lifetime_as<int>(std::declval<void*>())), int*>);
+static_assert(std::is_same_v<decltype(std::start_lifetime_as<int>(std::declval<const void*>())), const int*>);
+static_assert(std::is_same_v<decltype(std::start_lifetime_as<int>(std::declval<volatile void*>())), volatile int*>);
+static_assert(
+    std::is_same_v<decltype(std::start_lifetime_as<int>(std::declval<const volatile void*>())), const volatile int*>);
+
+void test_cv_qualifiers() {
+  alignas(Trivial) unsigned char buffer[sizeof(Trivial)];
+  void* p                  = buffer;
+  const void* cp           = buffer;
+  volatile void* vp        = buffer;
+  const volatile void* cvp = buffer;
+
+  Trivial* ptr                  = std::start_lifetime_as<Trivial>(p);
+  const Trivial* cptr           = std::start_lifetime_as<Trivial>(cp);
+  volatile Trivial* vptr        = std::start_lifetime_as<Trivial>(vp);
+  const volatile Trivial* cvptr = std::start_lifetime_as<Trivial>(cvp);
+
+  assert(static_cast<void*>(ptr) == buffer);
+  assert(static_cast<const void*>(cptr) == buffer);
+  assert(static_cast<volatile void*>(vptr) == buffer);
+  assert(static_cast<const volatile void*>(cvptr) == buffer);
----------------
frederick-vs-ja wrote:

IIUC these assertions doesn't verify that `start_lifetime_as` starts lifetime of objects (and reuses object representations) at all.

I think we definitely want to verify the values of re-created objects.

https://github.com/llvm/llvm-project/pull/196286