[libcxx-commits] [libcxx] [libc++] LWG4324: `unique_ptr<void>::operator*` is not SFINAE-friendly (PR #190919)

[A. Jiang via libcxx-commits]

https://github.com/frederick-vs-ja updated https://github.com/llvm/llvm-project/pull/190919

>From 1cd318a4595b220b917c8c8483b8bcdb480d3b69 Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34 at live.cn>
Date: Wed, 8 Apr 2026 14:56:03 +0800
Subject: [PATCH 1/2] [libc++] LWG4324: `unique_ptr<void>::operator*` is not
 SFINAE-friendly

---
 libcxx/docs/Status/Cxx2cIssues.csv                  |  2 +-
 libcxx/include/__memory/unique_ptr.h                |  7 +++++++
 .../dereference.single.pass.cpp                     | 13 +++++++++++++
 3 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/libcxx/docs/Status/Cxx2cIssues.csv b/libcxx/docs/Status/Cxx2cIssues.csv
index 75972e0b2874c..94584349e7282 100644
--- a/libcxx/docs/Status/Cxx2cIssues.csv
+++ b/libcxx/docs/Status/Cxx2cIssues.csv
@@ -280,7 +280,7 @@
 "`LWG4259 <https://wg21.link/LWG4259>`__","P1148R0 changed the return values of searching functions of ``std::basic_string`` on some platforms","2026-03 (Croydon)","","","`#189822 <https://github.com/llvm/llvm-project/issues/189822>`__",""
 "`LWG4290 <https://wg21.link/LWG4290>`__","Missing *Mandates* clauses on ``is_sufficiently_aligned``","2026-03 (Croydon)","","","`#189823 <https://github.com/llvm/llvm-project/issues/189823>`__",""
 "`LWG4314 <https://wg21.link/LWG4314>`__","Missing move in ``mdspan`` layout ``mapping::operator()``","2026-03 (Croydon)","","","`#189824 <https://github.com/llvm/llvm-project/issues/189824>`__",""
-"`LWG4324 <https://wg21.link/LWG4324>`__","``unique_ptr<void>::operator*`` is not SFINAE-friendly","2026-03 (Croydon)","","","`#189825 <https://github.com/llvm/llvm-project/issues/189825>`__",""
+"`LWG4324 <https://wg21.link/LWG4324>`__","``unique_ptr<void>::operator*`` is not SFINAE-friendly","2026-03 (Croydon)","|Complete|","23","`#189825 <https://github.com/llvm/llvm-project/issues/189825>`__",""
 "`LWG4325 <https://wg21.link/LWG4325>`__","``std::indirect``'s ``operator==`` still does not support incomplete types","2026-03 (Croydon)","","","`#189826 <https://github.com/llvm/llvm-project/issues/189826>`__",""
 "`LWG4339 <https://wg21.link/LWG4339>`__","``task``'s coroutine frame may be released late","2026-03 (Croydon)","","","`#189827 <https://github.com/llvm/llvm-project/issues/189827>`__",""
 "`LWG4347 <https://wg21.link/LWG4347>`__","``task``'s stop source is always created","2026-03 (Croydon)","","","`#189828 <https://github.com/llvm/llvm-project/issues/189828>`__",""
diff --git a/libcxx/include/__memory/unique_ptr.h b/libcxx/include/__memory/unique_ptr.h
index 6a4ec0a466ba7..8fba2a4a1509d 100644
--- a/libcxx/include/__memory/unique_ptr.h
+++ b/libcxx/include/__memory/unique_ptr.h
@@ -117,6 +117,12 @@ struct __unique_ptr_deleter_sfinae<_Deleter&> {
   typedef false_type __enable_rval_overload;
 };
 
+template <class, class = void>
+inline const bool __can_dereference = false;
+
+template <class _Tp>
+inline const bool __can_dereference<_Tp, decltype((void)*std::declval<_Tp>())> = true;
+
 #if defined(_LIBCPP_ABI_ENABLE_UNIQUE_PTR_TRIVIAL_ABI)
 #  define _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI __attribute__((__trivial_abi__))
 #else
@@ -258,6 +264,7 @@ class _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI unique_ptr {
     return *this;
   }
 
+  template <class _Ptr = pointer, __enable_if_t<__can_dereference<_Ptr>, int> = 0>
   [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 __add_lvalue_reference_t<_Tp> operator*() const
       _NOEXCEPT_(_NOEXCEPT_(*std::declval<pointer>())) {
     return *__ptr_;
diff --git a/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/dereference.single.pass.cpp b/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/dereference.single.pass.cpp
index 4063190838a27..4db17a4805df0 100644
--- a/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/dereference.single.pass.cpp
+++ b/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/dereference.single.pass.cpp
@@ -12,10 +12,13 @@
 
 // test op*()
 
+// XFAIL: FROZEN-CXX03-HEADERS-FIXME
+
 #include <cassert>
 #include <memory>
 #include <utility>
 #include <vector>
+#include <type_traits>
 
 #include "test_macros.h"
 
@@ -31,6 +34,16 @@ struct Deleter {
 };
 #endif
 
+template <class, class = void>
+struct can_dereference : std::false_type {};
+template <class T>
+struct can_dereference<T, decltype((void)*std::declval<T>())> : std::true_type {};
+
+static_assert(can_dereference<std::unique_ptr<int> >::value, "");
+static_assert(can_dereference<const std::unique_ptr<int>&>::value, "");
+static_assert(!can_dereference<std::unique_ptr<void> >::value, "");
+static_assert(!can_dereference<const std::unique_ptr<void>&>::value, "");
+
 TEST_CONSTEXPR_CXX23 bool test() {
   {
     std::unique_ptr<int> p(new int(3));

>From 1589f1d83a324438e6a1f89a1d6b993398b3f34a Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34 at live.cn>
Date: Sat, 9 May 2026 19:29:04 +0800
Subject: [PATCH 2/2] Sort header inclusions per @Zingam's suggestion

Co-authored-by: Hristo Hristov <zingam at outlook.com>
---
 .../unique.ptr.observers/dereference.single.pass.cpp            | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/dereference.single.pass.cpp b/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/dereference.single.pass.cpp
index 4db17a4805df0..b9d8a1b563b6c 100644
--- a/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/dereference.single.pass.cpp
+++ b/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/dereference.single.pass.cpp
@@ -16,9 +16,9 @@
 
 #include <cassert>
 #include <memory>
+#include <type_traits>
 #include <utility>
 #include <vector>
-#include <type_traits>
 
 #include "test_macros.h"