[libcxx] implement LWG4148: unique_ptr::operator* should not allow dangling references

Author: elhewaty

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",""

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_; }

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;
+}