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

[via libcxx-commits]

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

>From 21bcffd15c474ad000a2280050ae1e781a80c95a 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          |  7 ++++++
 .../test/libcxx/memory/unique_ptr.verify.cpp  | 24 +++++++++++++++++++
 3 files changed, 32 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 1ec23dfabd5ea..d639e4a1417a3 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)","","",""
 "`LWG4144 <https://wg21.link/LWG4144>`__","Disallow ``unique_ptr<T&, D>``","2024-11 (Wrocław)","","",""
 "`LWG4147 <https://wg21.link/LWG4147>`__","Precondition on ``inplace_vector::emplace``","2024-11 (Wrocław)","","",""
-"`LWG4148 <https://wg21.link/LWG4148>`__","``unique_ptr::operator*`` should not allow dangling references","2024-11 (Wrocław)","","",""
+"`LWG4148 <https://wg21.link/LWG4148>`__","``unique_ptr::operator*`` should not allow dangling references","2024-11 (Wrocław)","|Complete|","21",""
 "`LWG4153 <https://wg21.link/LWG4153>`__","Fix extra ""-1"" for ``philox_engine::max()``","2024-11 (Wrocław)","","",""
 "`LWG4154 <https://wg21.link/LWG4154>`__","The Mandates for ``std::packaged_task``'s constructor from a callable entity should consider decaying","2024-11 (Wrocław)","","",""
 "`LWG4157 <https://wg21.link/LWG4157>`__","The resolution of LWG3465 was damaged by P2167R3","2024-11 (Wrocław)","","20",""
diff --git a/libcxx/include/__memory/unique_ptr.h b/libcxx/include/__memory/unique_ptr.h
index e8ef386f7f9fc..d4e9ed325e9eb 100644
--- a/libcxx/include/__memory/unique_ptr.h
+++ b/libcxx/include/__memory/unique_ptr.h
@@ -262,6 +262,13 @@ class _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS 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 !defined(_LIBCPP_CLANG_VER) || _LIBCPP_CLANG_VER != 1800
+    // TODO: use reference_converts_from_temporary_v once implemented.
+    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..550d57455f4ed
--- /dev/null
+++ b/libcxx/test/libcxx/memory/unique_ptr.verify.cpp
@@ -0,0 +1,24 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <memory>
+
+#include <memory>
+
+struct deleter {
+  using pointer = long*;
+  void operator()(pointer) const {}
+};
+
+int main(int argc, char**) {
+  long l = 0;
+  std::unique_ptr<const int, deleter> p(&l);
+  int i =
+      *p; // expect-error {{static assertion failed due to requirement '!__reference_converts_from_temporary(const int &, long &)': Reference type _Tp must not convert from a temporary object}}
+  return 0;
+}