[libcxx-commits] [libcxx] [libc++] Implement std::make_from_tuple mandates: If tuple_size_v<remove_reference_t<Tuple>> is 1, then reference_constructs_from_temporary_v<T, decltype(get<0>(declval<Tuple>()))> is false. (PR #152867)

[via libcxx-commits]

https://github.com/yronglin created https://github.com/llvm/llvm-project/pull/152867

Implement the following wording for `std::make_from_tuple`.
```
Mandates: If tuple_size_v<remove_reference_t<Tuple>> is 1, then reference_constructs_from_temporary_v<T, decltype(get<0>(declval<Tuple>()))> is false.
```

>From 7d345a59e0a3b3b0be34634d2431cc6f4835dbd7 Mon Sep 17 00:00:00 2001
From: yronglin <yronglin777 at gmail.com>
Date: Sun, 10 Aug 2025 00:47:22 +0800
Subject: [PATCH] [libc++] Implement std::make_from_tuple mandates: If
 tuple_size_v<remove_reference_t<Tuple>> is 1, then
 reference_constructs_from_temporary_v<T, decltype(get<0>(declval<Tuple>()))>
 is false.

Signed-off-by: yronglin <yronglin777 at gmail.com>
---
 libcxx/include/tuple                          | 14 ++++++++---
 .../tuple.apply/make_from_tuple.verify.cpp    | 23 +++++++++++++++++++
 2 files changed, 34 insertions(+), 3 deletions(-)
 create mode 100644 libcxx/test/std/utilities/tuple/tuple.tuple/tuple.apply/make_from_tuple.verify.cpp

diff --git a/libcxx/include/tuple b/libcxx/include/tuple
index be30ab5b2173d..9988470102266 100644
--- a/libcxx/include/tuple
+++ b/libcxx/include/tuple
@@ -1432,6 +1432,7 @@ inline _LIBCPP_HIDE_FROM_ABI constexpr _Tp __make_from_tuple_impl(_Tuple&& __t,
     enable_if_t<is_constructible_v<_Tp, decltype(std::get<_Idx>(std::forward<_Tuple>(__t)))...>> * = nullptr)
     _LIBCPP_NOEXCEPT_RETURN(_Tp(std::get<_Idx>(std::forward<_Tuple>(__t))...))
 #endif // _LIBCPP_STD_VER >= 20
+#undef _LIBCPP_NOEXCEPT_RETURN
 
 template <class _Tp, class _Tuple,
           class _Seq = make_index_sequence<tuple_size_v<remove_reference_t<_Tuple>>>, class = void>
@@ -1451,10 +1452,17 @@ template <class _Tp, class _Tuple>
 #else
 template <class _Tp, class _Tuple, class = enable_if_t<__can_make_from_tuple<_Tp, _Tuple>>> // strengthen
 #endif // _LIBCPP_STD_VER >= 20
+
 inline _LIBCPP_HIDE_FROM_ABI constexpr _Tp make_from_tuple(_Tuple&& __t)
-    _LIBCPP_NOEXCEPT_RETURN(std::__make_from_tuple_impl<_Tp>(
-        std::forward<_Tuple>(__t), make_index_sequence<tuple_size_v<remove_reference_t<_Tuple>>>()))
-#    undef _LIBCPP_NOEXCEPT_RETURN
+  noexcept(noexcept(std::__make_from_tuple_impl<_Tp>(std::forward<_Tuple>(__t),
+                    make_index_sequence<tuple_size_v<remove_reference_t<_Tuple>>>()))) {
+  if constexpr (tuple_size_v<remove_reference_t<_Tuple>> == 1) {
+    static_assert(!__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");
+  }
+  return std::__make_from_tuple_impl<_Tp>(
+        std::forward<_Tuple>(__t), make_index_sequence<tuple_size_v<remove_reference_t<_Tuple>>>());
+}
 
 #  endif // _LIBCPP_STD_VER >= 17
 
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.apply/make_from_tuple.verify.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.apply/make_from_tuple.verify.cpp
new file mode 100644
index 0000000000000..0ca16088964f7
--- /dev/null
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.apply/make_from_tuple.verify.cpp
@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14
+
+// <tuple>
+
+// template <class T, class Tuple> constexpr T make_from_tuple(Tuple&&);
+// Mandates: If tuple_size_v<remove_reference_t<Tuple>> is 1, then reference_constructs_from_temporary_v<T, decltype(get<0>(declval<Tuple>()))> is false.
+
+#include <tuple>
+#include <utility>
+
+void test() {
+  // expected-error@*:* {{static assertion failed}}
+  // expected-error@*:* {{returning reference to local temporary object}}
+  std::ignore = std::make_from_tuple<const int&>(std::tuple<char>{});
+}