[libc++] Remove guards for builtins for reference binding traits (#166288)

We're only supporting Clang 20+ and Apple Clang 17 now, where these
builtins are universally implemented.

Author: frederick-vs-ja

libcxx/include/__type_traits/reference_constructs_from_temporary.h

@@ -18,7 +18,7 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER >= 23 && __has_builtin(__reference_constructs_from_temporary)
+#if _LIBCPP_STD_VER >= 23
 
 template <class _Tp, class _Up>
 struct _LIBCPP_NO_SPECIALIZATIONS reference_constructs_from_temporary

libcxx/include/__type_traits/reference_converts_from_temporary.h

@@ -18,7 +18,7 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER >= 23 && __has_builtin(__reference_converts_from_temporary)
+#if _LIBCPP_STD_VER >= 23
 
 template <class _Tp, class _Up>
 struct _LIBCPP_NO_SPECIALIZATIONS reference_converts_from_temporary

libcxx/include/tuple

@@ -1443,7 +1443,7 @@ template <class _Tp, class _Tuple, class = enable_if_t<__can_make_from_tuple<_Tp
 inline _LIBCPP_HIDE_FROM_ABI constexpr _Tp make_from_tuple(_Tuple&& __t)
   noexcept(noexcept(std::__make_from_tuple_impl<_Tp>(std::forward<_Tuple>(__t),
                     make_index_sequence<tuple_size_v<remove_reference_t<_Tuple>>>()))) {
-#if _LIBCPP_STD_VER >= 23 && __has_builtin(__reference_constructs_from_temporary)
+#if _LIBCPP_STD_VER >= 23
   if constexpr (tuple_size_v<remove_reference_t<_Tuple>> == 1) {
     static_assert(!std::reference_constructs_from_temporary_v<_Tp, decltype(std::get<0>(std::declval<_Tuple>()))>,
                   "Attempted construction of reference element binds to a temporary whose lifetime has ended");

libcxx/test/libcxx/type_traits/no_specializations.verify.cpp

@@ -154,14 +154,10 @@ SPECIALIZE_UTT(is_unbounded_array);     // expected-error 2 {{cannot be speciali
 #  endif
 
 #  if TEST_STD_VER >= 23
-SPECIALIZE_UTT(is_implicit_lifetime); // expected-error 2 {{cannot be specialized}}
-SPECIALIZE_UTT(is_scoped_enum);       // expected-error 2 {{cannot be specialized}}
-#    if __has_builtin(__reference_constructs_from_temporary)
+SPECIALIZE_UTT(is_implicit_lifetime);                // expected-error 2 {{cannot be specialized}}
+SPECIALIZE_UTT(is_scoped_enum);                      // expected-error 2 {{cannot be specialized}}
 SPECIALIZE_BTT(reference_constructs_from_temporary); // expected-error 2 {{cannot be specialized}}
-#    endif
-#    if __has_builtin(__reference_converts_from_temporary)
-SPECIALIZE_BTT(reference_converts_from_temporary); // expected-error 2 {{cannot be specialized}}
-#    endif
+SPECIALIZE_BTT(reference_converts_from_temporary);   // expected-error 2 {{cannot be specialized}}
 #  endif
 
 #  if TEST_STD_VER >= 26

libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/reference_constructs_from_temporary.pass.cpp

@@ -8,9 +8,6 @@
 
 // REQUIRES: std-at-least-c++23
 
-// These compilers don't support std::reference_converts_from_temporary yet.
-// UNSUPPORTED: apple-clang-16, clang-19.1
-
 // <type_traits>
 
 // template<class T, class U> struct reference_constructs_from_temporary;

libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/reference_converts_from_temporary.pass.cpp

@@ -8,9 +8,6 @@
 
 // REQUIRES: std-at-least-c++23
 
-// These compilers don't support std::reference_converts_from_temporary yet.
-// UNSUPPORTED: apple-clang-16, clang-19.1
-
 // <type_traits>
 
 // template<class T, class U> struct reference_converts_from_temporary;

libcxx/test/std/utilities/tuple/tuple.tuple/tuple.apply/make_from_tuple.verify.cpp

@@ -19,11 +19,7 @@
 #include "test_macros.h"
 
 void test() {
-  // FreeBSD ci use clang 19.1.1, which hasn't implement __reference_constructs_from_temporary.
-  // The static_assert inner std::make_from_tuple will not triggered.
-#if __has_builtin(__reference_constructs_from_temporary)
   // expected-error@*:* {{static assertion failed}}
-#endif
 
   // Turns to an error since C++26 (Disallow Binding a Returned Glvalue to a Temporary https://wg21.link/P2748R5).
 #if TEST_STD_VER >= 26