[libc++][tuple.apply] Implement P2255R2 make_from_tuple part.

[yronglin]

Implement P2255R2 tuple.apply part 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.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: None (yronglin)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/152867.diff

2 Files Affected:

• (modified) libcxx/include/tuple (+11-3)
• (added) libcxx/test/std/utilities/tuple/tuple.tuple/tuple.apply/make_from_tuple.verify.cpp (+23)

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>{});
+}

[philnik777]

If we don't close this, please update the PR description to be maybe one line, definitely not four.

[yronglin]

If we don't close this, please update the PR description to be maybe one line, definitely not four.

Sure! It was changed to a more shortly title now.

[philnik777]

Suggested change

	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");
	}
	static_assert(tuple_size_v<remove_reference_t<_Tuple>> != 1 || !__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");

[frederick-vs-ja]

No... this will break std::make_from_tuple<int>(std::tuple<>{}) as we can't call std::get<0>() with tuple<>.