[libcxx-commits] [libcxx] 617b446 - [libc++] Implement P1789R3: Library Support for Expansion Statements (#167184)
via libcxx-commits
libcxx-commits at lists.llvm.org
Thu Dec 25 17:07:18 PST 2025
Author: Matthias Wippich
Date: 2025-12-26T09:07:13+08:00
New Revision: 617b446176b2014a288ee14a5f4ce0ac1d0283a2
URL: https://github.com/llvm/llvm-project/commit/617b446176b2014a288ee14a5f4ce0ac1d0283a2
DIFF: https://github.com/llvm/llvm-project/commit/617b446176b2014a288ee14a5f4ce0ac1d0283a2.diff
LOG: [libc++] Implement P1789R3: Library Support for Expansion Statements (#167184)
[P1789R3](https://isocpp.org/files/papers/P1789R3.pdf) was accepted for
C++26 through LWG motion 14 at the 2025 Kona meeting. This patch
implements it, along with tests and documentation changes.
Closes #167268
---------
Co-authored-by: Tsche <che at pydong.org>
Added:
libcxx/test/std/utilities/intseq/intseq.binding/structured_binding.pass.cpp
libcxx/test/std/utilities/intseq/intseq.binding/tuple_interface.compile.pass.cpp
libcxx/test/std/utilities/intseq/intseq.binding/tuple_interface.verify.cpp
Modified:
libcxx/docs/FeatureTestMacroTable.rst
libcxx/docs/ReleaseNotes/22.rst
libcxx/docs/Status/Cxx2cPapers.csv
libcxx/include/__utility/integer_sequence.h
libcxx/include/utility
libcxx/include/version
libcxx/test/libcxx/utilities/intseq/nodiscard.verify.cpp
libcxx/test/std/language.support/support.limits/support.limits.general/utility.version.compile.pass.cpp
libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp
libcxx/utils/generate_feature_test_macro_components.py
Removed:
################################################################################
diff --git a/libcxx/docs/FeatureTestMacroTable.rst b/libcxx/docs/FeatureTestMacroTable.rst
index 4a82461b80534..32911d0f64449 100644
--- a/libcxx/docs/FeatureTestMacroTable.rst
+++ b/libcxx/docs/FeatureTestMacroTable.rst
@@ -474,6 +474,8 @@ Status
---------------------------------------------------------- -----------------
``__cpp_lib_inplace_vector`` *unimplemented*
---------------------------------------------------------- -----------------
+ ``__cpp_lib_integer_sequence`` ``202511L``
+ ---------------------------------------------------------- -----------------
``__cpp_lib_is_sufficiently_aligned`` ``202411L``
---------------------------------------------------------- -----------------
``__cpp_lib_is_virtual_base_of`` ``202406L``
diff --git a/libcxx/docs/ReleaseNotes/22.rst b/libcxx/docs/ReleaseNotes/22.rst
index 82414e88ac5fb..0d1a1fbc00f2c 100644
--- a/libcxx/docs/ReleaseNotes/22.rst
+++ b/libcxx/docs/ReleaseNotes/22.rst
@@ -53,6 +53,7 @@ Implemented Papers
- P3168R2: Give ``std::optional`` Range Support (`Github <https://llvm.org/PR105430>`__)
- P3567R2: ``flat_meow`` Fixes (`Github <https://llvm.org/PR162022>`__)
- P3836R2: Make ``optional<T&>`` trivially copyable (`Github <https://llvm.org/PR171275>`__)
+- P1789R3: Library Support for Expansion Statements (`Github <https://llvm.org/PR167184>`__)
Improvements and New Features
-----------------------------
diff --git a/libcxx/docs/Status/Cxx2cPapers.csv b/libcxx/docs/Status/Cxx2cPapers.csv
index 6a77ec83b61b8..9bb5d2bda3d4d 100644
--- a/libcxx/docs/Status/Cxx2cPapers.csv
+++ b/libcxx/docs/Status/Cxx2cPapers.csv
@@ -169,7 +169,7 @@
"`P3819R0 <https://wg21.link/P3819R0>`__","Remove ``evaluation_exception()`` from contract-violation handling for C++26","2025-11 (Kona)","","","`#171280 <https://github.com/llvm/llvm-project/issues/171280>`__",""
"`P3612R1 <https://wg21.link/P3612R1>`__","Harmonize proxy-reference operations (LWG 3638 and 4187)","2025-11 (Kona)","","","`#171281 <https://github.com/llvm/llvm-project/issues/171281>`__",""
"`P3778R0 <https://wg21.link/P3778R0>`__","Fix for ``type_order`` template definition","2025-11 (Kona)","","","`#171284 <https://github.com/llvm/llvm-project/issues/171284>`__",""
-"`P1789R3 <https://wg21.link/P1789R3>`__","Library Support for Expansion Statements","2025-11 (Kona)","","","`#167268 <https://github.com/llvm/llvm-project/issues/167268>`__",""
+"`P1789R3 <https://wg21.link/P1789R3>`__","Library Support for Expansion Statements","2025-11 (Kona)","|Complete|","22","`#167268 <https://github.com/llvm/llvm-project/issues/167268>`__",""
"`P3922R1 <https://wg21.link/P3922R1>`__","Missing deduction guide from ``simd::mask`` to ``simd::vec``","2025-11 (Kona)","","","`#171285 <https://github.com/llvm/llvm-project/issues/171285>`__",""
"`P3878R1 <https://wg21.link/P3878R1>`__","Standard library hardening should not use the 'observe' semantic","2025-11 (Kona)","","","`#171286 <https://github.com/llvm/llvm-project/issues/171286>`__",""
"`P3887R1 <https://wg21.link/P3887R1>`__","Make ``when_all`` a Ronseal Algorithm","2025-11 (Kona)","","","`#171289 <https://github.com/llvm/llvm-project/issues/171289>`__",""
diff --git a/libcxx/include/__utility/integer_sequence.h b/libcxx/include/__utility/integer_sequence.h
index 9450d526ed32c..a84f572c3339c 100644
--- a/libcxx/include/__utility/integer_sequence.h
+++ b/libcxx/include/__utility/integer_sequence.h
@@ -11,6 +11,8 @@
#include <__config>
#include <__cstddef/size_t.h>
+#include <__tuple/tuple_element.h>
+#include <__tuple/tuple_size.h>
#include <__type_traits/is_integral.h>
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -70,6 +72,30 @@ _LIBCPP_HIDE_FROM_ABI constexpr void __for_each_index_sequence(index_sequence<_I
}
# endif // _LIBCPP_STD_VER >= 20
+# if _LIBCPP_STD_VER >= 26
+// [intseq.binding], structured binding support
+template <class _Tp, _Tp... _Indices>
+struct tuple_size<integer_sequence<_Tp, _Indices...>> : integral_constant<size_t, sizeof...(_Indices)> {};
+
+template <size_t _Ip, class _Tp, _Tp... _Indices>
+struct tuple_element<_Ip, integer_sequence<_Tp, _Indices...>> {
+ static_assert(_Ip < sizeof...(_Indices), "Index out of bounds in std::tuple_element<> (std::integer_sequence)");
+ using type _LIBCPP_NODEBUG = _Tp;
+};
+
+template <size_t _Ip, class _Tp, _Tp... _Indices>
+struct tuple_element<_Ip, const integer_sequence<_Tp, _Indices...>> {
+ static_assert(_Ip < sizeof...(_Indices), "Index out of bounds in std::tuple_element<> (const std::integer_sequence)");
+ using type _LIBCPP_NODEBUG = _Tp;
+};
+
+template <size_t _Ip, class _Tp, _Tp... _Indices>
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp get(integer_sequence<_Tp, _Indices...>) noexcept {
+ static_assert(_Ip < sizeof...(_Indices), "Index out of bounds in std::get<> (std::integer_sequence)");
+ return _Indices...[_Ip];
+}
+# endif // _LIBCPP_STD_VER >= 26
+
# endif // _LIBCPP_STD_VER >= 14
_LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/include/utility b/libcxx/include/utility
index bc4eaf6a0cd02..1b19243afca1b 100644
--- a/libcxx/include/utility
+++ b/libcxx/include/utility
@@ -216,6 +216,18 @@ template<size_t N>
template<class... T>
using index_sequence_for = make_index_sequence<sizeof...(T)>;
+template<class T, T... Values> // C++26
+ struct tuple_size<integer_sequence<T, Values...>>;
+
+template<size_t I, class T, T... Values> // C++26
+ struct tuple_element<I, integer_sequence<T, Values...>>;
+
+template<size_t I, class T, T... Values> // C++26
+ struct tuple_element<I, const integer_sequence<T, Values...>>;
+
+template<size_t I, class T, T... Values> // C++26
+ constexpr T get(integer_sequence<T, Values...>) noexcept;
+
template<class T, class U=T>
constexpr T exchange(T& obj, U&& new_value) // constexpr in C++17, noexcept in C++23
noexcept(is_nothrow_move_constructible<T>::value && is_nothrow_assignable<T&, U>::value);
diff --git a/libcxx/include/version b/libcxx/include/version
index f959c0ab227ac..ab781466f5ed5 100644
--- a/libcxx/include/version
+++ b/libcxx/include/version
@@ -141,7 +141,8 @@ __cpp_lib_incomplete_container_elements 201505L <forward_list> <
__cpp_lib_inplace_vector 202406L <inplace_vector>
__cpp_lib_int_pow2 202002L <bit>
__cpp_lib_integer_comparison_functions 202002L <utility>
-__cpp_lib_integer_sequence 201304L <utility>
+__cpp_lib_integer_sequence 202511L <utility>
+ 201304L // C++14
__cpp_lib_integral_constant_callable 201304L <type_traits>
__cpp_lib_interpolate 201902L <cmath> <numeric>
__cpp_lib_invoke 201411L <functional>
@@ -583,6 +584,8 @@ __cpp_lib_void_t 201411L <type_traits>
// # define __cpp_lib_generate_random 202403L
// # define __cpp_lib_hazard_pointer 202306L
// # define __cpp_lib_inplace_vector 202406L
+# undef __cpp_lib_integer_sequence
+# define __cpp_lib_integer_sequence 202511L
# define __cpp_lib_is_sufficiently_aligned 202411L
# if __has_builtin(__builtin_is_virtual_base_of)
# define __cpp_lib_is_virtual_base_of 202406L
diff --git a/libcxx/test/libcxx/utilities/intseq/nodiscard.verify.cpp b/libcxx/test/libcxx/utilities/intseq/nodiscard.verify.cpp
index 4ca52ace08dd3..811759f37823c 100644
--- a/libcxx/test/libcxx/utilities/intseq/nodiscard.verify.cpp
+++ b/libcxx/test/libcxx/utilities/intseq/nodiscard.verify.cpp
@@ -14,8 +14,13 @@
#include <utility>
+#include "test_macros.h"
+
void test() {
std::integer_sequence<int, 49, 82, 94> seq;
seq.size(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+#if TEST_STD_VER >= 26
+ get<0>(seq); // expected-warning {{ignoring return value of function}}
+#endif
}
diff --git a/libcxx/test/std/language.support/support.limits/support.limits.general/utility.version.compile.pass.cpp b/libcxx/test/std/language.support/support.limits/support.limits.general/utility.version.compile.pass.cpp
index b882a5df04ae3..1d82ef9ec0e86 100644
--- a/libcxx/test/std/language.support/support.limits/support.limits.general/utility.version.compile.pass.cpp
+++ b/libcxx/test/std/language.support/support.limits/support.limits.general/utility.version.compile.pass.cpp
@@ -432,8 +432,8 @@
# ifndef __cpp_lib_integer_sequence
# error "__cpp_lib_integer_sequence should be defined in c++26"
# endif
-# if __cpp_lib_integer_sequence != 201304L
-# error "__cpp_lib_integer_sequence should have the value 201304L in c++26"
+# if __cpp_lib_integer_sequence != 202511L
+# error "__cpp_lib_integer_sequence should have the value 202511L in c++26"
# endif
# if !defined(_LIBCPP_VERSION)
diff --git a/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp b/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp
index 28fae95c1c1f7..b3b424a1d77ce 100644
--- a/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp
+++ b/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp
@@ -7156,8 +7156,8 @@
# ifndef __cpp_lib_integer_sequence
# error "__cpp_lib_integer_sequence should be defined in c++26"
# endif
-# if __cpp_lib_integer_sequence != 201304L
-# error "__cpp_lib_integer_sequence should have the value 201304L in c++26"
+# if __cpp_lib_integer_sequence != 202511L
+# error "__cpp_lib_integer_sequence should have the value 202511L in c++26"
# endif
# ifndef __cpp_lib_integral_constant_callable
diff --git a/libcxx/test/std/utilities/intseq/intseq.binding/structured_binding.pass.cpp b/libcxx/test/std/utilities/intseq/intseq.binding/structured_binding.pass.cpp
new file mode 100644
index 0000000000000..650e0d651ea7d
--- /dev/null
+++ b/libcxx/test/std/utilities/intseq/intseq.binding/structured_binding.pass.cpp
@@ -0,0 +1,54 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+// <utility>
+
+// template<size_t I, class T, T... Values>
+// struct tuple_element<I, integer_sequence<T, Values...>>;
+// template<size_t I, class T, T... Values>
+// struct tuple_element<I, const integer_sequence<T, Values...>>;
+// template<size_t I, class T, T... Values>
+// constexpr T get(integer_sequence<T, Values...>) noexcept;
+
+#include <cassert>
+#include <utility>
+
+constexpr bool test() {
+ auto [elt0, elt1, elt2, elt3] = std::make_index_sequence<4>();
+
+ assert(elt0 == 0);
+ assert(elt1 == 1);
+ assert(elt2 == 2);
+ assert(elt3 == 3);
+
+// TODO: remove this macro guard once GCC16 is available
+#if __cpp_structured_bindings >= 202411L
+ []<typename...> {
+ auto [... empty] = std::make_index_sequence<0>();
+ static_assert(sizeof...(empty) == 0);
+
+ auto [... size4] = std::make_index_sequence<4>();
+ static_assert(sizeof...(size4) == 4);
+
+ assert(size4...[0] == 0);
+ assert(size4...[1] == 1);
+ assert(size4...[2] == 2);
+ assert(size4...[3] == 3);
+ }();
+#endif
+
+ return true;
+}
+
+int main(int, char**) {
+ test();
+ static_assert(test());
+ return 0;
+}
diff --git a/libcxx/test/std/utilities/intseq/intseq.binding/tuple_interface.compile.pass.cpp b/libcxx/test/std/utilities/intseq/intseq.binding/tuple_interface.compile.pass.cpp
new file mode 100644
index 0000000000000..026f20eb8c09f
--- /dev/null
+++ b/libcxx/test/std/utilities/intseq/intseq.binding/tuple_interface.compile.pass.cpp
@@ -0,0 +1,58 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+// <utility>
+
+// template<class T, T... Values>
+// struct tuple_size<integer_sequence<T, Values...>>;
+// template<size_t I, class T, T... Values>
+// struct tuple_element<I, integer_sequence<T, Values...>>;
+// template<size_t I, class T, T... Values>
+// struct tuple_element<I, const integer_sequence<T, Values...>>;
+// template<size_t I, class T, T... Values>
+// constexpr T get(integer_sequence<T, Values...>) noexcept;
+
+#include <cassert>
+#include <concepts>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+void test() {
+ // std::tuple_size_v
+ using empty = std::integer_sequence<int>;
+ static_assert(std::tuple_size_v<empty> == 0);
+ static_assert(std::tuple_size_v<const empty> == 0);
+
+ using size4 = std::integer_sequence<int, 9, 8, 7, 2>;
+ static_assert(std::tuple_size_v<size4> == 4);
+ static_assert(std::tuple_size_v<const size4> == 4);
+
+ // std::tuple_element_t
+ static_assert(std::is_same_v<std::tuple_element_t<0, size4>, int>);
+ static_assert(std::is_same_v<std::tuple_element_t<1, size4>, int>);
+ static_assert(std::is_same_v<std::tuple_element_t<2, size4>, int>);
+ static_assert(std::is_same_v<std::tuple_element_t<3, size4>, int>);
+
+ static_assert(std::is_same_v<std::tuple_element_t<0, const size4>, int>);
+ static_assert(std::is_same_v<std::tuple_element_t<1, const size4>, int>);
+ static_assert(std::is_same_v<std::tuple_element_t<2, const size4>, int>);
+ static_assert(std::is_same_v<std::tuple_element_t<3, const size4>, int>);
+
+ // std::get
+ constexpr static size4 seq4;
+ constexpr std::same_as<int> decltype(auto) elt0 = get<0>(seq4);
+ static_assert(elt0 == 9);
+ static_assert(get<1>(seq4) == 8);
+ static_assert(get<2>(seq4) == 7);
+ static_assert(get<3>(seq4) == 2);
+
+ static_assert(noexcept(get<0>(seq4)));
+}
diff --git a/libcxx/test/std/utilities/intseq/intseq.binding/tuple_interface.verify.cpp b/libcxx/test/std/utilities/intseq/intseq.binding/tuple_interface.verify.cpp
new file mode 100644
index 0000000000000..a1c894e985729
--- /dev/null
+++ b/libcxx/test/std/utilities/intseq/intseq.binding/tuple_interface.verify.cpp
@@ -0,0 +1,34 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+// <utility>
+
+// template<size_t I, class T, T... Values>
+// struct tuple_element<I, integer_sequence<T, Values...>>;
+// template<size_t I, class T, T... Values>
+// struct tuple_element<I, const integer_sequence<T, Values...>>;
+// template<size_t I, class T, T... Values>
+// constexpr T get(integer_sequence<T, Values...>) noexcept;
+
+// Expect failures for tuple_element and get with empty integer_sequence
+
+#include <utility>
+
+void test() {
+ // expected-error-re@*:* {{static assertion failed{{.*}}Index out of bounds in std::tuple_element<> (std::integer_sequence)}}
+ using test1 = std::tuple_element_t<0, std::integer_sequence<int>>;
+ // expected-error-re@*:* {{static assertion failed{{.*}}Index out of bounds in std::tuple_element<> (const std::integer_sequence)}}
+ using test2 = std::tuple_element_t<0, const std::integer_sequence<int>>;
+
+ std::integer_sequence<int> empty;
+ // expected-error-re@*:* {{static assertion failed{{.*}}Index out of bounds in std::get<> (std::integer_sequence)}}
+ // expected-error-re@*:* {{invalid index 0 for pack {{.*}} of size 0}}
+ (void)std::get<0>(empty);
+}
diff --git a/libcxx/utils/generate_feature_test_macro_components.py b/libcxx/utils/generate_feature_test_macro_components.py
index ff965aaa4ff89..f61d6f991cb15 100644
--- a/libcxx/utils/generate_feature_test_macro_components.py
+++ b/libcxx/utils/generate_feature_test_macro_components.py
@@ -764,7 +764,10 @@ def add_version_header(tc):
},
{
"name": "__cpp_lib_integer_sequence",
- "values": {"c++14": 201304},
+ "values": {
+ "c++14": 201304,
+ "c++26": 202511, # P1789R3 Library Support for Expansion Statements
+ },
"headers": ["utility"],
},
{
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