[libcxx-commits] [libcxx] [libc++] Disable optional<T&> constructor for optional<T> (PR #194446)

[William Tran-Viet via libcxx-commits]

https://github.com/smallp-o-p updated https://github.com/llvm/llvm-project/pull/194446

>From ffac919419ebfbc5d2f17cf3d5d420d7e262c400 Mon Sep 17 00:00:00 2001
From: William Tran-Viet <wtranviet at proton.me>
Date: Mon, 27 Apr 2026 15:36:51 -0400
Subject: [PATCH 1/2] Disable optional<T&> constructor for optional<T>

---
 libcxx/include/optional                              | 12 ++++++++----
 .../optional.object.ctor/optional_U.pass.cpp         |  5 +++++
 .../optional/optional.object/optional_helper_types.h | 10 ++++++++++
 3 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/libcxx/include/optional b/libcxx/include/optional
index 9eed886aa4636..f1a2c5567a49a 100644
--- a/libcxx/include/optional
+++ b/libcxx/include/optional
@@ -893,6 +893,10 @@ private:
   template <class _Up>
   constexpr static bool __libcpp_opt_ref_ctor_deleted =
       is_lvalue_reference_v<_Tp> && reference_constructs_from_temporary_v<_Tp, _Up>;
+
+  template <class _Up>
+  constexpr static bool __libcpp_enable_opt_ref_ctor =
+      is_lvalue_reference_v<_Tp> && !reference_constructs_from_temporary_v<_Tp, _Up>;
 #    endif
 
   // LWG2756: conditionally explicit conversion from _Up
@@ -1046,8 +1050,8 @@ public:
 
   // optional(optional<U>& rhs)
   template <class _Up>
-    requires(!__libcpp_opt_ref_ctor_deleted<_Up>) && (!is_same_v<remove_cvref_t<_Tp>, optional<_Up>>) &&
-            (!is_same_v<_Tp&, _Up>) && is_constructible_v<_Tp&, _Up&>
+    requires __libcpp_enable_opt_ref_ctor<_Up> && (!is_same_v<remove_cvref_t<_Tp>, optional<_Up>>) &&
+             (!is_same_v<_Tp&, _Up>) && is_constructible_v<_Tp&, _Up&>
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_Up&, _Tp&>)
       optional(optional<_Up>& __rhs) noexcept(is_nothrow_constructible_v<_Tp&, _Up&>) {
     this->__construct_from(__rhs);
@@ -1078,8 +1082,8 @@ public:
 
   // optional(const optional<U>&&)
   template <class _Up>
-    requires(!__libcpp_opt_ref_ctor_deleted<_Up>) && (!is_same_v<remove_cvref_t<_Tp>, optional<_Up>>) &&
-            (!is_same_v<_Tp&, _Up>) && is_constructible_v<_Tp&, _Up>
+    requires __libcpp_enable_opt_ref_ctor<_Up> && (!is_same_v<remove_cvref_t<_Tp>, optional<_Up>>) &&
+             (!is_same_v<_Tp&, _Up>) && is_constructible_v<_Tp&, _Up>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 explicit(!is_convertible_v<const _Up, _Tp&>)
       optional(const optional<_Up>&& __v) noexcept(is_nothrow_constructible_v<_Tp&, const _Up>) {
     this->__construct_from(std::move(__v));
diff --git a/libcxx/test/std/utilities/optional/optional.object/optional.object.ctor/optional_U.pass.cpp b/libcxx/test/std/utilities/optional/optional.object/optional.object.ctor/optional_U.pass.cpp
index 39a80efa475f7..ce86193f5ea4e 100644
--- a/libcxx/test/std/utilities/optional/optional.object/optional.object.ctor/optional_U.pass.cpp
+++ b/libcxx/test/std/utilities/optional/optional.object/optional.object.ctor/optional_U.pass.cpp
@@ -164,6 +164,11 @@ int main(int, char**) {
     test<Z>(std::move(rhs), true);
   }
 
+#if TEST_STD_VER >= 26
+  // GH: #194415
+  static_assert(!std::is_constructible_v<std::optional<int>, std::optional<LValueOnly<int>>&>);
+#endif
+
   static_assert(!(std::is_constructible<optional<X>, optional<Z>>::value), "");
 
 #if TEST_STD_VER >= 26
diff --git a/libcxx/test/std/utilities/optional/optional.object/optional_helper_types.h b/libcxx/test/std/utilities/optional/optional.object/optional_helper_types.h
index 704d99acb15e9..e119a32bd5522 100644
--- a/libcxx/test/std/utilities/optional/optional.object/optional_helper_types.h
+++ b/libcxx/test/std/utilities/optional/optional.object/optional_helper_types.h
@@ -51,4 +51,14 @@ struct ReferenceConversionThrows {
   }
 };
 
+template <typename T>
+struct LValueOnly {
+  T val{};
+
+  constexpr operator T&() & noexcept { return val; }
+  constexpr operator T&() const&  = delete;
+  constexpr operator T&() &&      = delete;
+  constexpr operator T&() const&& = delete;
+};
+
 #endif

>From ef1f154c4029db3148a561e3db9df612c278ce60 Mon Sep 17 00:00:00 2001
From: William Tran-Viet <wtranviet at proton.me>
Date: Mon, 27 Apr 2026 16:01:28 -0400
Subject: [PATCH 2/2] Add regression test for optional(const optional<U>&&)

---
 .../optional.object.ctor/const_optional_U.pass.cpp     |  2 ++
 .../optional/optional.object/optional_helper_types.h   | 10 ++++++++++
 2 files changed, 12 insertions(+)

diff --git a/libcxx/test/std/utilities/optional/optional.object/optional.object.ctor/const_optional_U.pass.cpp b/libcxx/test/std/utilities/optional/optional.object/optional.object.ctor/const_optional_U.pass.cpp
index 597110bcb54fe..a3561cff0bc67 100644
--- a/libcxx/test/std/utilities/optional/optional.object/optional.object.ctor/const_optional_U.pass.cpp
+++ b/libcxx/test/std/utilities/optional/optional.object/optional.object.ctor/const_optional_U.pass.cpp
@@ -151,6 +151,8 @@ constexpr bool test_ref() {
     assert(*o2 == 1);
   }
 
+  static_assert(!std::is_constructible_v<std::optional<int>, const std::optional<ConstRValueOnly<int>>&&>);
+
   return true;
 }
 #endif
diff --git a/libcxx/test/std/utilities/optional/optional.object/optional_helper_types.h b/libcxx/test/std/utilities/optional/optional.object/optional_helper_types.h
index e119a32bd5522..2b28c25568204 100644
--- a/libcxx/test/std/utilities/optional/optional.object/optional_helper_types.h
+++ b/libcxx/test/std/utilities/optional/optional.object/optional_helper_types.h
@@ -61,4 +61,14 @@ struct LValueOnly {
   constexpr operator T&() const&& = delete;
 };
 
+template <typename T>
+struct ConstRValueOnly {
+  mutable T val{};
+
+  constexpr operator T&() &      = delete;
+  constexpr operator T&() const& = delete;
+  constexpr operator T&() &&     = delete;
+  constexpr operator T&() const&& { return val; };
+};
+
 #endif