[libc++] Use __reference_constructs_from_temporary if eligible

[frederick-vs-ja]

Currently, libc++'s <tuple> is using the deprecated __reference_binds_to_temporary intrinsic. This PR starts to use __reference_constructs_from_temporary if possible.

It seems that __reference_constructs_from_temporary should be used via an internal type traits provided in
<__type_traits/reference_constructs_from_temporary.h>. But given the old intrinsic was directly used, this PR doesn't switch to the current convention yet.

P2255R2 is related. Although the paper indicated that constructors of tuple should be deleted in such a case.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: A. Jiang (frederick-vs-ja)

Changes

Currently, libc++'s <tuple> is using the deprecated __reference_binds_to_temporary intrinsic. This PR starts to use __reference_constructs_from_temporary if possible.

It seems that __reference_constructs_from_temporary should be used via an internal type traits provided in
<__type_traits/reference_constructs_from_temporary.h>. But given the old intrinsic was directly used, this PR doesn't switch to the current convention yet.

P2255R2 is related. Although the paper indicated that constructors of tuple should be deleted in such a case.

---

Full diff: https://github.com/llvm/llvm-project/pull/141916.diff

3 Files Affected:

• (modified) libcxx/include/tuple (+3-1)
• (modified) libcxx/test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.verify.cpp (+1-1)
• (modified) libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp (+6-5)

diff --git a/libcxx/include/tuple b/libcxx/include/tuple
index 8dd62ae624f5e..5aef90ac19cfe 100644
--- a/libcxx/include/tuple
+++ b/libcxx/include/tuple
@@ -310,7 +310,9 @@ class __tuple_leaf {
 
   template <class _Tp>
   static _LIBCPP_HIDE_FROM_ABI constexpr bool __can_bind_reference() {
-#    if __has_keyword(__reference_binds_to_temporary)
+#    if __has_builtin(__reference_constructs_from_temporary)
+    return !__reference_constructs_from_temporary(_Hp, _Tp);
+#    else if __has_keyword(__reference_binds_to_temporary)
     return !__reference_binds_to_temporary(_Hp, _Tp);
 #    else
     return true;
diff --git a/libcxx/test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.verify.cpp b/libcxx/test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.verify.cpp
index 4a6e3095c1019..df068bf35f2e5 100644
--- a/libcxx/test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.verify.cpp
+++ b/libcxx/test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.verify.cpp
@@ -40,7 +40,7 @@ void F(typename CannotDeduce<std::tuple<Args...>>::type const&) {}
 
 
 void f() {
-#if TEST_HAS_BUILTIN_IDENTIFIER(__reference_binds_to_temporary)
+#if TEST_HAS_BUILTIN_IDENTIFIER(__reference_constructs_from_temporary)
   // Test that we emit our diagnostic from the library.
   // expected-error@tuple:* 8 {{Attempted construction of reference element binds to a temporary whose lifetime has ended}}
 
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp
index 463816929353b..1da967d89718f 100644
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp
@@ -18,12 +18,13 @@
 #include <cassert>
 #include "test_macros.h"
 
-#if TEST_HAS_BUILTIN_IDENTIFIER(__reference_binds_to_temporary)
-# define ASSERT_REFERENCE_BINDS_TEMPORARY(...) static_assert(__reference_binds_to_temporary(__VA_ARGS__), "")
-# define ASSERT_NOT_REFERENCE_BINDS_TEMPORARY(...) static_assert(!__reference_binds_to_temporary(__VA_ARGS__), "")
+#if TEST_HAS_BUILTIN_IDENTIFIER(__reference_constructs_from_temporary)
+#  define ASSERT_REFERENCE_BINDS_TEMPORARY(...) static_assert(__reference_constructs_from_temporary(__VA_ARGS__), "")
+#  define ASSERT_NOT_REFERENCE_BINDS_TEMPORARY(...)                                                                    \
+    static_assert(!__reference_constructs_from_temporary(__VA_ARGS__), "")
 #else
-# define ASSERT_REFERENCE_BINDS_TEMPORARY(...) static_assert(true, "")
-# define ASSERT_NOT_REFERENCE_BINDS_TEMPORARY(...) static_assert(true, "")
+#  define ASSERT_REFERENCE_BINDS_TEMPORARY(...) static_assert(true, "")
+#  define ASSERT_NOT_REFERENCE_BINDS_TEMPORARY(...) static_assert(true, "")
 #endif
 
 template <class Tp>

[frederick-vs-ja]

@philnik777 @Zingam @H-G-Hristov Can we land this? (As this seems blocking proper deprecation warning of __reference_binds_to_temporary.)