[libcxx-commits] [libcxx] [libc++][test] Test LWG3819: `reference_meows_from_temporary` should not use `is_meowible` (PR #143474)

[A. Jiang via libcxx-commits]

https://github.com/frederick-vs-ja created https://github.com/llvm/llvm-project/pull/143474

Actually, this LWG issue is not "Nothing To Do". But the changes need to be done by compilers, and library implementations can hardly do anything.

Closes #105079.

>From d5322bcf2f1f596afb9623da0e6789c841971b75 Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34 at live.cn>
Date: Wed, 4 Jun 2025 20:23:58 +0800
Subject: [PATCH] [libc++][test] Test LWG3819: `reference_meows_from_temporary`
 should...

not use `is_meowible`

Actually, this LWG issue is not "Nothing To Do". But the changes need to
be done by compilers, and library implementations can hardly do
anything.
---
 libcxx/docs/Status/Cxx23Issues.csv            |  2 +-
 .../meta/meta.unary/meta.unary.prop/common.h  |  8 ++++++++
 ...ference_constructs_from_temporary.pass.cpp | 20 +++++++++++++++++++
 ...reference_converts_from_temporary.pass.cpp | 20 +++++++++++++++++++
 4 files changed, 49 insertions(+), 1 deletion(-)

diff --git a/libcxx/docs/Status/Cxx23Issues.csv b/libcxx/docs/Status/Cxx23Issues.csv
index b5c88611559ef..5c3d8f1c477f6 100644
--- a/libcxx/docs/Status/Cxx23Issues.csv
+++ b/libcxx/docs/Status/Cxx23Issues.csv
@@ -261,7 +261,7 @@
 "`LWG3733 <https://wg21.link/LWG3733>`__","``ranges::to`` misuses ``cpp17-input-iterator``","2023-02 (Issaquah)","|Complete|","17",""
 "`LWG3742 <https://wg21.link/LWG3742>`__","``deque::prepend_range`` needs to permute","2023-02 (Issaquah)","","",""
 "`LWG3790 <https://wg21.link/LWG3790>`__","`P1467 <https://wg21.link/P1467>`__ accidentally changed ``nexttoward``'s signature","2023-02 (Issaquah)","","",""
-"`LWG3819 <https://wg21.link/LWG3819>`__","``reference_meows_from_temporary`` should not use ``is_meowible``","2023-02 (Issaquah)","","",""
+"`LWG3819 <https://wg21.link/LWG3819>`__","``reference_meows_from_temporary`` should not use ``is_meowible``","2023-02 (Issaquah)","|Nothing To Do|","",""
 "`LWG3821 <https://wg21.link/LWG3821>`__","``uses_allocator_construction_args`` should have overload for ``pair-like``","2023-02 (Issaquah)","|Complete|","18",""
 "`LWG3834 <https://wg21.link/LWG3834>`__","Missing ``constexpr`` for ``std::intmax_t`` math functions in ``<cinttypes>``","2023-02 (Issaquah)","","",""
 "`LWG3839 <https://wg21.link/LWG3839>`__","``range_formatter``'s ``set_separator``, ``set_brackets``, and ``underlying`` functions should be ``noexcept``","2023-02 (Issaquah)","|Complete|","17",""
diff --git a/libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/common.h b/libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/common.h
index b51444915a5c5..4fba34e3817be 100644
--- a/libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/common.h
+++ b/libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/common.h
@@ -85,6 +85,14 @@ class ExplicitConversionRef {
   explicit operator int&();
 };
 
+struct NonMovable {
+  NonMovable(NonMovable&&) = delete;
+};
+
+struct ConvertsFromNonMovable {
+  ConvertsFromNonMovable(NonMovable);
+};
+
 #endif
 
 #endif // TEST_META_UNARY_COMP_COMMON_H
diff --git a/libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/reference_constructs_from_temporary.pass.cpp b/libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/reference_constructs_from_temporary.pass.cpp
index 5b3753c67381f..4a09e34b37b1b 100644
--- a/libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/reference_constructs_from_temporary.pass.cpp
+++ b/libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/reference_constructs_from_temporary.pass.cpp
@@ -58,6 +58,7 @@ constexpr bool test() {
   assert((std::is_constructible_v<const int&, ConvertsToRef<long, long&>>));
   test_reference_constructs_from_temporary<const int&, ConvertsToRef<long, long&>, true>();
 #ifndef TEST_COMPILER_GCC
+  // TODO: Remove this guard once https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120529 gets fixed.
   test_reference_constructs_from_temporary<const int&, ConvertsToRefPrivate<long, long&>, false>();
 #endif
 
@@ -66,6 +67,25 @@ constexpr bool test() {
 
   test_reference_constructs_from_temporary<const int&, long, true>();
 
+#if defined(TEST_COMPILER_GCC) || (defined(TEST_CLANG_VER) && TEST_CLANG_VER >= 2100)
+  // TODO: Remove this guard once no supported Clang is affected by https://github.com/llvm/llvm-project/issues/114344.
+
+  // Test function references.
+  test_reference_constructs_from_temporary<void (&)(), void(), false>();
+  test_reference_constructs_from_temporary<void (&&)(), void(), false>();
+
+  // Test cv-qualification dropping for scalar prvalues. LWG3819 also covers this.
+  test_reference_constructs_from_temporary<int&&, const int, true>();
+  test_reference_constructs_from_temporary<int&&, volatile int, true>();
+#endif
+
+#if defined(TEST_CLANG_VER) && TEST_CLANG_VER >= 2100
+  // TODO: Remove this guard once supported Clang and GCC have LWG3819 implemented.
+
+  // Test LWG3819: reference_meows_from_temporary should not use is_meowible.
+  test_reference_constructs_from_temporary<ConvertsFromNonMovable&&, NonMovable, true>();
+#endif
+
   // Additional checks
   test_reference_constructs_from_temporary<const Base&, Derived, true>();
   test_reference_constructs_from_temporary<int&&, int, true>();
diff --git a/libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/reference_converts_from_temporary.pass.cpp b/libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/reference_converts_from_temporary.pass.cpp
index 849e286c8cdab..1b199a497b372 100644
--- a/libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/reference_converts_from_temporary.pass.cpp
+++ b/libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/reference_converts_from_temporary.pass.cpp
@@ -58,6 +58,7 @@ constexpr bool test() {
   assert((std::is_constructible_v<const int&, ConvertsToRef<long, long&>>));
   test_reference_converts_from_temporary<const int&, ConvertsToRef<long, long&>, true>();
 #ifndef TEST_COMPILER_GCC
+  // TODO: Remove this guard once https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120529 gets fixed.
   test_reference_converts_from_temporary<const int&, ConvertsToRefPrivate<long, long&>, false>();
 #endif
 
@@ -66,6 +67,25 @@ constexpr bool test() {
 
   test_reference_converts_from_temporary<const int&, long, true>();
 
+#if defined(TEST_COMPILER_GCC) || (defined(TEST_CLANG_VER) && TEST_CLANG_VER >= 2100)
+  // TODO: Remove this guard once no supported Clang is affected by https://github.com/llvm/llvm-project/issues/114344.
+
+  // Test function references.
+  test_reference_converts_from_temporary<void (&)(), void(), false>();
+  test_reference_converts_from_temporary<void (&&)(), void(), false>();
+
+  // Test cv-qualification dropping for scalar prvalues. LWG3819 also covers this.
+  test_reference_converts_from_temporary<int&&, const int, true>();
+  test_reference_converts_from_temporary<int&&, volatile int, true>();
+#endif
+
+#if defined(TEST_CLANG_VER) && TEST_CLANG_VER >= 2100
+  // TODO: Remove this guard once supported Clang and GCC have LWG3819 implemented.
+
+  // Test LWG3819: reference_meows_from_temporary should not use is_meowible.
+  test_reference_converts_from_temporary<ConvertsFromNonMovable&&, NonMovable, true>();
+#endif
+
   // Additional checks
   test_reference_converts_from_temporary<const Base&, Derived, true>();
   test_reference_converts_from_temporary<int&&, int, true>();