[libc++] Use __reference_constructs_from_temporary if eligible (#141916)

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.

---------

Co-authored-by: Nikolas Klauser <[email protected]>

Author: frederick-vs-ja

libcxx/include/__type_traits/reference_constructs_from_temporary.h

@@ -30,6 +30,15 @@ _LIBCPP_NO_SPECIALIZATIONS inline constexpr bool reference_constructs_from_tempo
 
 #endif
 
+#if __has_builtin(__reference_constructs_from_temporary)
+template <class _Tp, class _Up>
+inline const bool __reference_constructs_from_temporary_v = __reference_constructs_from_temporary(_Tp, _Up);
+#else
+// TODO(LLVM 22): Remove this as all supported compilers should have __reference_constructs_from_temporary implemented.
+template <class _Tp, class _Up>
+inline const bool __reference_constructs_from_temporary_v = __reference_binds_to_temporary(_Tp, _Up);
+#endif
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___TYPE_TRAITS_REFERENCE_CONSTRUCTS_FROM_TEMPORARY_H

libcxx/include/tuple

@@ -258,6 +258,7 @@ template <class... Types>
 #  include <__type_traits/maybe_const.h>
 #  include <__type_traits/nat.h>
 #  include <__type_traits/negation.h>
+#  include <__type_traits/reference_constructs_from_temporary.h>
 #  include <__type_traits/remove_cv.h>
 #  include <__type_traits/remove_cvref.h>
 #  include <__type_traits/remove_reference.h>
@@ -308,15 +309,6 @@ template <size_t _Ip, class _Hp, bool>
 class __tuple_leaf {
   _Hp __value_;
 
-  template <class _Tp>
-  static _LIBCPP_HIDE_FROM_ABI constexpr bool __can_bind_reference() {
-#    if __has_keyword(__reference_binds_to_temporary)
-    return !__reference_binds_to_temporary(_Hp, _Tp);
-#    else
-    return true;
-#    endif
-  }
-
 public:
   _LIBCPP_CONSTEXPR_SINCE_CXX14 __tuple_leaf& operator=(const __tuple_leaf&) = delete;
 
@@ -346,15 +338,15 @@ public:
   _LIBCPP_HIDE_FROM_ABI
   _LIBCPP_CONSTEXPR_SINCE_CXX14 explicit __tuple_leaf(_Tp&& __t) noexcept(is_nothrow_constructible<_Hp, _Tp>::value)
       : __value_(std::forward<_Tp>(__t)) {
-    static_assert(__can_bind_reference<_Tp&&>(),
+    static_assert(!__reference_constructs_from_temporary_v<_Hp, _Tp&&>,
                   "Attempted construction of reference element binds to a temporary whose lifetime has ended");
   }
 
   template <class _Tp, class _Alloc>
   _LIBCPP_HIDE_FROM_ABI
   _LIBCPP_CONSTEXPR_SINCE_CXX14 explicit __tuple_leaf(integral_constant<int, 0>, const _Alloc&, _Tp&& __t)
       : __value_(std::forward<_Tp>(__t)) {
-    static_assert(__can_bind_reference<_Tp&&>(),
+    static_assert(!__reference_constructs_from_temporary_v<_Hp, _Tp&&>,
                   "Attempted construction of reference element binds to a temporary whose lifetime has ended");
   }
 

libcxx/test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.verify.cpp

@@ -38,9 +38,7 @@ template <class T> struct CannotDeduce {
 template <class ...Args>
 void F(typename CannotDeduce<std::tuple<Args...>>::type const&) {}
 
-
 void f() {
-#if TEST_HAS_BUILTIN_IDENTIFIER(__reference_binds_to_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}}
 
@@ -73,8 +71,4 @@ void f() {
     std::tuple<std::string &&> t2("hello"); // expected-note {{requested here}}
     std::tuple<std::string &&> t3(std::allocator_arg, alloc, "hello"); // expected-note {{requested here}}
   }
-#else
-#error force failure
-// expected-error@-1 {{force failure}}
-#endif
 }

libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp

@@ -18,12 +18,14 @@
 #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(__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, "")
+// TODO(LLVM 22): Remove this as all support compilers should have __reference_constructs_from_temporary implemented.
+#  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__), "")
 #endif
 
 template <class Tp>