[gcc r16-8602] libstdc++: Implement structured binding support for integer_sequence
Tomasz Kaminski
tkaminsk@gcc.gnu.org
Tue Apr 14 10:56:20 GMT 2026
https://gcc.gnu.org/g:6159bfcf8b5852417519cf4181bf0ff8907ac0ca
commit r16-8602-g6159bfcf8b5852417519cf4181bf0ff8907ac0ca
Author: Matthias Wippich <mfwippich@gmail.com>
Date: Thu Apr 9 04:41:31 2026 +0200
libstdc++: Implement structured binding support for integer_sequence
This implements P1789R3 Library Support for Expansion Statements.
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p1789r3.pdf
libstdc++-v3/ChangeLog:
* include/bits/utility.h (std::tuple_size, std::tuple_element):
Add partial specializations for integer_sequence.
(std::get(integer_sequence<_Tp, _Idx...>)): Define.
* include/bits/version.def (integer_sequence): Bump to 202511L.
* include/bits/version.h: Regenerate.
* testsuite/20_util/integer_sequence/tuple_access.cc: New test.
* testsuite/20_util/integer_sequence/tuple_access_neg.cc: New test.
* testsuite/experimental/feat-cxx14.cc: Updated check for
__cpp_lib_integer_sequence value.
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Co-authored-by: Ivan Lazaric <ivan.lazaric1@gmail.com>
Signed-off-by: Matthias Wippich <mfwippich@gmail.com>
Diff:
---
libstdc++-v3/include/bits/utility.h | 36 ++++++
libstdc++-v3/include/bits/version.def | 5 +
libstdc++-v3/include/bits/version.h | 7 +-
.../20_util/integer_sequence/tuple_access.cc | 143 +++++++++++++++++++++
.../20_util/integer_sequence/tuple_access_neg.cc | 22 ++++
libstdc++-v3/testsuite/experimental/feat-cxx14.cc | 4 +-
6 files changed, 214 insertions(+), 3 deletions(-)
diff --git a/libstdc++-v3/include/bits/utility.h b/libstdc++-v3/include/bits/utility.h
index 93e9e9f9dba2..74ae04d6bd2e 100644
--- a/libstdc++-v3/include/bits/utility.h
+++ b/libstdc++-v3/include/bits/utility.h
@@ -481,6 +481,42 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
static constexpr size_t size() noexcept { return sizeof...(_Idx); }
};
+#if __glibcxx_integer_sequence >= 202511L // C++ >= 26
+ /** @brief Structured binding support for `integer_sequence`
+
+ * @since C++26
+ * @{
+ */
+ /// Structured binding support
+ template<typename _Tp, _Tp... _Idx>
+ struct tuple_size<integer_sequence<_Tp, _Idx...>>
+ : integral_constant<size_t, sizeof...(_Idx)> { };
+
+ template<size_t __i, class _Tp, _Tp... _Idx>
+ struct tuple_element<__i, integer_sequence<_Tp, _Idx...>>
+ {
+ static_assert(__i < sizeof...(_Idx));
+ using type = _Tp;
+ };
+
+ template<size_t __i, class _Tp, _Tp... _Idx>
+ struct tuple_element<__i, const integer_sequence<_Tp, _Idx...>>
+ {
+ static_assert(__i < sizeof...(_Idx));
+ using type = _Tp;
+ };
+
+ template<size_t __i, class _Tp, _Tp... _Idx>
+ [[nodiscard]]
+ constexpr _Tp
+ get(integer_sequence<_Tp, _Idx...>) noexcept
+ {
+ static_assert(__i < sizeof...(_Idx));
+ return _Idx...[__i];
+ }
+ /// @}
+#endif // __glibcxx_integer_sequence >= 202511L
+
/// Alias template make_integer_sequence
template<typename _Tp, _Tp _Num>
using make_integer_sequence
diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
index b88d9c3483f8..7aaf150d0875 100644
--- a/libstdc++-v3/include/bits/version.def
+++ b/libstdc++-v3/include/bits/version.def
@@ -184,6 +184,11 @@ ftms = {
ftms = {
name = integer_sequence;
+ values = {
+ v = 202511;
+ cxxmin = 26;
+ extra_cond = "__cpp_pack_indexing";
+ };
values = {
v = 201304;
cxxmin = 14;
diff --git a/libstdc++-v3/include/bits/version.h b/libstdc++-v3/include/bits/version.h
index 24ec0175e1e4..b247662f089e 100644
--- a/libstdc++-v3/include/bits/version.h
+++ b/libstdc++-v3/include/bits/version.h
@@ -186,7 +186,12 @@
#undef __glibcxx_want_exchange_function
#if !defined(__cpp_lib_integer_sequence)
-# if (__cplusplus >= 201402L)
+# if (__cplusplus > 202302L) && (__cpp_pack_indexing)
+# define __glibcxx_integer_sequence 202511L
+# if defined(__glibcxx_want_all) || defined(__glibcxx_want_integer_sequence)
+# define __cpp_lib_integer_sequence 202511L
+# endif
+# elif (__cplusplus >= 201402L)
# define __glibcxx_integer_sequence 201304L
# if defined(__glibcxx_want_all) || defined(__glibcxx_want_integer_sequence)
# define __cpp_lib_integer_sequence 201304L
diff --git a/libstdc++-v3/testsuite/20_util/integer_sequence/tuple_access.cc b/libstdc++-v3/testsuite/20_util/integer_sequence/tuple_access.cc
new file mode 100644
index 000000000000..afa644e7dab7
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/integer_sequence/tuple_access.cc
@@ -0,0 +1,143 @@
+// { dg-do compile { target c++26 } }
+
+#include <utility>
+#include <tuple>
+#include <testsuite_hooks.h>
+
+#if __cpp_lib_integer_sequence < 202511L
+# error "Feature-test macro __cpp_lib_integer_sequence is incorrect"
+#endif
+
+constexpr auto
+destructure_sum(auto seq)
+{
+ auto [...elems] = seq;
+ return (0 + ... + elems);
+}
+
+using IS1 = std::make_index_sequence<10>;
+static_assert( std::tuple_size_v<IS1> == 10 );
+static_assert( std::is_same_v<std::tuple_element_t<3, IS1>, std::size_t> );
+static_assert( std::get<7>(IS1{}) == 7 );
+static_assert( destructure_sum(IS1{}) == 45 );
+static_assert( noexcept(get<0>(IS1{})) );
+
+using IS2 = std::integer_sequence<int, 42, 101, -13>;
+static_assert( std::tuple_size_v<IS2> == 3 );
+static_assert( std::is_same_v<std::tuple_element_t<1, IS2>, int> );
+static_assert( std::get<2>(IS2{}) == -13 );
+static_assert( destructure_sum(IS2{}) == 130 );
+static_assert( noexcept(get<0>(IS2{})) );
+
+using IS3 = std::integer_sequence<char>;
+static_assert( std::tuple_size_v<IS3> == 0 );
+
+using IS4 = std::integer_sequence<int, 1, 2>;
+static_assert( !std::is_constructible_v<std::pair<int, int>, IS4> );
+static_assert( !std::is_constructible_v<std::tuple<int, int>, IS4> );
+
+template<typename = void>
+constexpr bool
+test_basic()
+{
+ {
+ auto [...elems] = std::make_index_sequence<0>{};
+ static_assert( sizeof...(elems) == 0 );
+ }
+
+ {
+ auto [...elems] = std::integer_sequence<int, 3, 5, 7, 11>{};
+
+ static_assert( sizeof...(elems) == 4 );
+
+ VERIFY( elems...[0] == 3 );
+ VERIFY( elems...[1] == 5 );
+ VERIFY( elems...[2] == 7 );
+ VERIFY( elems...[3] == 11 );
+ }
+
+ {
+ static constexpr auto [...elems] = std::integer_sequence<short, 2, 4, 8, 16>{};
+
+ static_assert( sizeof...(elems) == 4 );
+
+ static_assert( elems...[0] == 2 );
+ static_assert( elems...[1] == 4 );
+ static_assert( elems...[2] == 8 );
+ static_assert( elems...[3] == 16 );
+ }
+
+ {
+ constexpr auto [...elems] = std::integer_sequence<int, 1, 8, 2, 11>{};
+
+ static_assert( sizeof...(elems) == 4 );
+
+ static_assert( elems...[0] == 1 );
+ static_assert( elems...[1] == 8 );
+ static_assert( elems...[2] == 2 );
+ static_assert( elems...[3] == 11 );
+ }
+
+ {
+ static constexpr auto&& [...elems] = std::integer_sequence<short, 2, 4, 8, 16>{};
+
+ static_assert( sizeof...(elems) == 4 );
+
+ static_assert( elems...[0] == 2 );
+ static_assert( elems...[1] == 4 );
+ static_assert( elems...[2] == 8 );
+ static_assert( elems...[3] == 16 );
+ }
+
+ /* Unimplemented, see PR117784
+ {
+ constexpr auto&& [...elems] = std::integer_sequence<int, 1, 8, 2, 11>{};
+
+ static_assert( sizeof...(elems) == 4 );
+
+ static_assert( elems...[0] == 1 );
+ static_assert( elems...[1] == 8 );
+ static_assert( elems...[2] == 2 );
+ static_assert( elems...[3] == 11 );
+ }
+ */
+
+ {
+ auto idx = 0;
+ static constexpr auto seq = std::make_index_sequence<4>{};
+ template for (constexpr auto elem : seq)
+ {
+ VERIFY( elem == idx );
+ ++idx;
+ }
+ VERIFY( idx == 4 );
+ }
+
+
+ /* Unimplemented, see PR117784
+ {
+ auto idx = 0;
+ constexpr auto seq = std::make_index_sequence<4>{};
+ template for (constexpr auto elem : seq)
+ {
+ VERIFY( elem == idx );
+ ++idx;
+ }
+ VERIFY( idx == 4 );
+ }
+
+ {
+ auto idx = 0;
+ template for (constexpr auto elem : std::make_index_sequence<4>{})
+ {
+ VERIFY( elem == idx );
+ ++idx;
+ }
+ VERIFY( idx == 4 );
+ }
+ */
+
+ return true;
+}
+
+static_assert( test_basic() );
diff --git a/libstdc++-v3/testsuite/20_util/integer_sequence/tuple_access_neg.cc b/libstdc++-v3/testsuite/20_util/integer_sequence/tuple_access_neg.cc
new file mode 100644
index 000000000000..8640114e7d3d
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/integer_sequence/tuple_access_neg.cc
@@ -0,0 +1,22 @@
+// { dg-do compile { target c++26 } }
+
+#include <utility>
+
+using empty = std::integer_sequence<int>;
+static_assert( std::is_same_v<std::tuple_element_t<0, empty>, int> ); // { dg-error "here" }
+static_assert( std::is_same_v<std::tuple_element_t<0, const empty>, int> ); // { dg-error "here" }
+
+using size4 = std::integer_sequence<int, 1, 9, 7, 15>;
+static_assert( std::is_same_v<std::tuple_element_t<4, size4>, int> ); // { dg-error "here" }
+static_assert( std::is_same_v<std::tuple_element_t<4, const size4>, int> ); // { dg-error "here" }
+
+void
+test()
+{
+ (void)std::get<0>(empty{}); // { dg-error "here" }
+ (void)std::get<4>(size4{}); // { dg-error "here" }
+}
+
+// { dg-error "static assertion failed" "" { target *-*-* } 0 }
+// { dg-error "cannot index an empty pack" "" { target *-*-* } 0 }
+// { dg-error "pack index '.' is out of range for pack of length" "" { target *-*-* } 0 }
diff --git a/libstdc++-v3/testsuite/experimental/feat-cxx14.cc b/libstdc++-v3/testsuite/experimental/feat-cxx14.cc
index c009062b55cd..4cc56c5f76ec 100644
--- a/libstdc++-v3/testsuite/experimental/feat-cxx14.cc
+++ b/libstdc++-v3/testsuite/experimental/feat-cxx14.cc
@@ -18,8 +18,8 @@
#ifndef __cpp_lib_integer_sequence
# error "__cpp_lib_integer_sequence"
-#elif __cpp_lib_integer_sequence != 201304
-# error "__cpp_lib_integer_sequence != 201304"
+#elif __cpp_lib_integer_sequence < 201304
+# error "__cpp_lib_integer_sequence < 201304"
#endif
#ifndef __cpp_lib_exchange_function
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