[libcxx-commits] [libcxx] [libc++] Resolve LWG4439 and LWG4300 (PR #174257)

[via libcxx-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libcxx

Author: William Tran-Viet (smallp-o-p)

<details>
<summary>Changes</summary>

Resolves #<!-- -->171340, #<!-- -->171402

- Add `swap` test for `optional<T&>` to ensure an ADL-found swap isn't selected.
- Drive-by: Update documentation for LWG4300 since it's already completed

---
Full diff: https://github.com/llvm/llvm-project/pull/174257.diff


2 Files Affected:

- (modified) libcxx/docs/Status/Cxx2cIssues.csv (+2-2) 
- (modified) libcxx/test/std/utilities/optional/optional.specalg/swap.pass.cpp (+21) 


``````````diff
diff --git a/libcxx/docs/Status/Cxx2cIssues.csv b/libcxx/docs/Status/Cxx2cIssues.csv
index 8d312cd688daa..651b8c891ec00 100644
--- a/libcxx/docs/Status/Cxx2cIssues.csv
+++ b/libcxx/docs/Status/Cxx2cIssues.csv
@@ -179,7 +179,7 @@
 "`LWG4294 <https://wg21.link/LWG4294>`__","``bitset(const CharT*)`` constructor needs to be constrained","2025-11 (Kona)","","","`#171336 <https://github.com/llvm/llvm-project/issues/171336>`__",""
 "`LWG4297 <https://wg21.link/LWG4297>`__","Missing ``permutable`` constraint for iterator overloads in Parallel Range Algorithms","2025-11 (Kona)","","","`#171338 <https://github.com/llvm/llvm-project/issues/171338>`__",""
 "`LWG4299 <https://wg21.link/LWG4299>`__","Missing *Mandates:* part in ``optional<T&>::transform``","2025-11 (Kona)","","","`#171339 <https://github.com/llvm/llvm-project/issues/171339>`__",""
-"`LWG4300 <https://wg21.link/LWG4300>`__","Missing *Returns:* element in ``optional<T&>::emplace``","2025-11 (Kona)","","","`#171340 <https://github.com/llvm/llvm-project/issues/171340>`__",""
+"`LWG4300 <https://wg21.link/LWG4300>`__","Missing *Returns:* element in ``optional<T&>::emplace``","2025-11 (Kona)","|Complete|","22","`#171340 <https://github.com/llvm/llvm-project/issues/171340>`__",""
 "`LWG4301 <https://wg21.link/LWG4301>`__","``condition_variable{_any}::wait_{for, until}`` should take timeout by value","2025-11 (Kona)","","","`#171341 <https://github.com/llvm/llvm-project/issues/171341>`__",""
 "`LWG4302 <https://wg21.link/LWG4302>`__","Problematic ``vector_sum_of_squares`` wording","2025-11 (Kona)","","","`#171342 <https://github.com/llvm/llvm-project/issues/171342>`__",""
 "`LWG4304 <https://wg21.link/LWG4304>`__","``std::optional<NonReturnable&>`` is ill-formed due to ``value_or``","2025-11 (Kona)","","","`#171343 <https://github.com/llvm/llvm-project/issues/171343>`__",""
@@ -237,7 +237,7 @@
 "`LWG4434 <https://wg21.link/LWG4434>`__","``meta::is_accessible`` does not need to consider incomplete *D*","2025-11 (Kona)","","","`#171399 <https://github.com/llvm/llvm-project/issues/171399>`__",""
 "`LWG4435 <https://wg21.link/LWG4435>`__","``meta::has_identifier`` doesn't handle all types","2025-11 (Kona)","","","`#171400 <https://github.com/llvm/llvm-project/issues/171400>`__",""
 "`LWG4438 <https://wg21.link/LWG4438>`__","Bad expression in [exec.when.all]","2025-11 (Kona)","","","`#171401 <https://github.com/llvm/llvm-project/issues/171401>`__",""
-"`LWG4439 <https://wg21.link/LWG4439>`__","``std::optional<T&>::swap`` possibly selects ADL-found ``swap``","2025-11 (Kona)","","","`#171402 <https://github.com/llvm/llvm-project/issues/171402>`__",""
+"`LWG4439 <https://wg21.link/LWG4439>`__","``std::optional<T&>::swap`` possibly selects ADL-found ``swap``","2025-11 (Kona)","|Complete|","22","`#171402 <https://github.com/llvm/llvm-project/issues/171402>`__",""
 "`LWG4440 <https://wg21.link/LWG4440>`__","Forward declarations of entities need also in entries","2025-11 (Kona)","","","`#171403 <https://github.com/llvm/llvm-project/issues/171403>`__",""
 "`LWG4441 <https://wg21.link/LWG4441>`__","``ranges::rotate`` do not handle sized-but-not-sized-sentinel ranges correctly","2025-11 (Kona)","","","`#171404 <https://github.com/llvm/llvm-project/issues/171404>`__",""
 "`LWG4442 <https://wg21.link/LWG4442>`__","Clarify ``expr`` and ``fn`` for ``meta::reflect_object`` and ``meta::reflect_function``","2025-11 (Kona)","","","`#171405 <https://github.com/llvm/llvm-project/issues/171405>`__",""
diff --git a/libcxx/test/std/utilities/optional/optional.specalg/swap.pass.cpp b/libcxx/test/std/utilities/optional/optional.specalg/swap.pass.cpp
index c757120a1c146..0d84c6026f12e 100644
--- a/libcxx/test/std/utilities/optional/optional.specalg/swap.pass.cpp
+++ b/libcxx/test/std/utilities/optional/optional.specalg/swap.pass.cpp
@@ -175,6 +175,26 @@ constexpr bool test_swap_ref() {
   }
   return true;
 }
+
+struct ADLSwap {
+  static inline bool called_adl_swap{false};
+
+  friend void swap(ADLSwap&, ADLSwap&) noexcept { called_adl_swap = true; }
+};
+
+// LWG4439: Verify that an ADL-found swap is not selected when calling optional<T&>::swap()
+void test_ref_swap_adl() {
+  ADLSwap t1{};
+  ADLSwap t2{};
+
+  std::optional<ADLSwap&> o{t1};
+  std::optional<ADLSwap&> o2{t2};
+
+  o.swap(o2);
+
+  assert(!ADLSwap::called_adl_swap);
+}
+
 #endif
 
 int main(int, char**)
@@ -185,6 +205,7 @@ int main(int, char**)
     static_assert(test_swap_ref<double>());
     test_swap_ref<int>();
     test_swap_ref<double>();
+    test_ref_swap_adl();
 #endif
     {
         optional<int> opt1;

``````````

</details>


https://github.com/llvm/llvm-project/pull/174257