[libcxx-commits] [libcxx] [libcxx] implement LWG4148: unique_ptr::operator* should not allow dangling references (PR #128213)

[Hristo Hristov via libcxx-commits]

https://github.com/Zingam updated https://github.com/llvm/llvm-project/pull/128213

>From 5939c3d356f852ce085691090eddc1c2843dbc48 Mon Sep 17 00:00:00 2001
From: Mohamed Atef <mohamedatef1698 at gmail.com>
Date: Fri, 21 Feb 2025 20:13:33 +0200
Subject: [PATCH] [libcxx] implement LWG4148: unique_ptr::operator* should not
 allow dangling references

---
 libcxx/docs/Status/Cxx2cIssues.csv            |  2 +-
 libcxx/include/__memory/unique_ptr.h          |  8 ++++++
 .../test/libcxx/memory/unique_ptr.verify.cpp  | 27 +++++++++++++++++++
 3 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100644 libcxx/test/libcxx/memory/unique_ptr.verify.cpp

diff --git a/libcxx/docs/Status/Cxx2cIssues.csv b/libcxx/docs/Status/Cxx2cIssues.csv
index 7bf7bc95c8281..952fbfdfcefd7 100644
--- a/libcxx/docs/Status/Cxx2cIssues.csv
+++ b/libcxx/docs/Status/Cxx2cIssues.csv
@@ -103,7 +103,7 @@
 "`LWG4142 <https://wg21.link/LWG4142>`__","``format_parse_context::check_dynamic_spec`` should require at least one type","2024-11 (Wrocław)","","","`#118358 <https://github.com/llvm/llvm-project/issues/118358>`__",""
 "`LWG4144 <https://wg21.link/LWG4144>`__","Disallow ``unique_ptr<T&, D>``","2024-11 (Wrocław)","","","`#118359 <https://github.com/llvm/llvm-project/issues/118359>`__",""
 "`LWG4147 <https://wg21.link/LWG4147>`__","Precondition on ``inplace_vector::emplace``","2024-11 (Wrocław)","","","`#118361 <https://github.com/llvm/llvm-project/issues/118361>`__",""
-"`LWG4148 <https://wg21.link/LWG4148>`__","``unique_ptr::operator*`` should not allow dangling references","2024-11 (Wrocław)","","","`#118362 <https://github.com/llvm/llvm-project/issues/118362>`__",""
+"`LWG4148 <https://wg21.link/LWG4148>`__","``unique_ptr::operator*`` should not allow dangling references","2024-11 (Wrocław)","|Complete|","22","`#118362 <https://github.com/llvm/llvm-project/issues/118362>`__",""
 "`LWG4153 <https://wg21.link/LWG4153>`__","Fix extra ""-1"" for ``philox_engine::max()``","2024-11 (Wrocław)","","","`#118363 <https://github.com/llvm/llvm-project/issues/118363>`__",""
 "`LWG4154 <https://wg21.link/LWG4154>`__","The Mandates for ``std::packaged_task``'s constructor from a callable entity should consider decaying","2024-11 (Wrocław)","","","`#118364 <https://github.com/llvm/llvm-project/issues/118364>`__",""
 "`LWG4157 <https://wg21.link/LWG4157>`__","The resolution of LWG3465 was damaged by P2167R3","2024-11 (Wrocław)","|Complete|","20","`#118365 <https://github.com/llvm/llvm-project/issues/118365>`__",""
diff --git a/libcxx/include/__memory/unique_ptr.h b/libcxx/include/__memory/unique_ptr.h
index eff24546cdc01..1aaa6a99a979c 100644
--- a/libcxx/include/__memory/unique_ptr.h
+++ b/libcxx/include/__memory/unique_ptr.h
@@ -265,6 +265,14 @@ class _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI unique_ptr {
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 __add_lvalue_reference_t<_Tp> operator*() const
       _NOEXCEPT_(_NOEXCEPT_(*std::declval<pointer>())) {
+    // TODO(LLVM-21): Remove this workaround
+#if __has_builtin(__reference_converts_from_temporary) ||                                                              \
+    (defined(_LIBCPP_CLANG_VER) &&                                                                                     \
+     ((!defined(__ANDROID__) && _LIBCPP_CLANG_VER >= 1901) || (defined(__ANDROID__) && _LIBCPP_CLANG_VER >= 2000)))
+    static_assert(
+        !__reference_converts_from_temporary(__add_lvalue_reference_t<_Tp>, decltype(*std::declval<pointer>())),
+        "Reference type _Tp must not convert from a temporary object");
+#endif
     return *__ptr_;
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 pointer operator->() const _NOEXCEPT { return __ptr_; }
diff --git a/libcxx/test/libcxx/memory/unique_ptr.verify.cpp b/libcxx/test/libcxx/memory/unique_ptr.verify.cpp
new file mode 100644
index 0000000000000..6bd3d8818431f
--- /dev/null
+++ b/libcxx/test/libcxx/memory/unique_ptr.verify.cpp
@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++11
+
+// <memory>
+
+#include <memory>
+
+struct deleter {
+  using pointer = long*;
+  void operator()(pointer) const {}
+};
+
+int main(int, char**) {
+  long l = 0;
+  std::unique_ptr<const int, deleter> p(&l);
+  // expected-error-re@*:* {{static assertion failed{{.*}}'!__reference_converts_from_temporary(const int &, long &)': Reference type _Tp must not convert from a temporary object}}
+  // expected-error@*:*{{returning reference to local temporary object}}
+  int i = *p; // expected-note {{requested here}}
+  return 0;
+}