[PATCH v2] libstdc++: Implement structured binding support for integer_sequence
Tomasz Kaminski
tkaminsk@redhat.com
Thu Apr 9 11:02:55 GMT 2026
On Thu, Apr 9, 2026 at 10:04 AM Tomasz Kamiński <tkaminsk@redhat.com> wrote:
> From: Matthias Wippich <mfwippich@gmail.com>
>
> 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 partrial 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.
>
> Co-authored-by: Ivan Lazaric <ivan.lazaric1@gmail.com>
> Signed-off-by: Matthias Wippich <mfwippich@gmail.com>
> ---
> v3:
> - updates changelog
> - fixes __cpp_lib_integer_sequence check in feat-cxx14.cc
> - replaces dg-excess-errors in tuple_access_neg.cc
> - add few testcasse to tuple_access.cc
>
> Testing on x86_64-linux. OK for trunk when all test passes?
>
All test passed (noting this also for myself as I have many patches in
flight).
>
> libstdc++-v3/include/bits/utility.h | 30 ++++
> libstdc++-v3/include/bits/version.def | 5 +
> libstdc++-v3/include/bits/version.h | 7 +-
> .../20_util/integer_sequence/tuple_access.cc | 143 ++++++++++++++++++
> .../integer_sequence/tuple_access_neg.cc | 22 +++
> .../testsuite/experimental/feat-cxx14.cc | 4 +-
> 6 files changed, 208 insertions(+), 3 deletions(-)
> create mode 100644
> libstdc++-v3/testsuite/20_util/integer_sequence/tuple_access.cc
> create mode 100644
> libstdc++-v3/testsuite/20_util/integer_sequence/tuple_access_neg.cc
>
> diff --git a/libstdc++-v3/include/bits/utility.h
> b/libstdc++-v3/include/bits/utility.h
> index 93e9e9f9dba..363aed478c5 100644
> --- a/libstdc++-v3/include/bits/utility.h
> +++ b/libstdc++-v3/include/bits/utility.h
> @@ -481,6 +481,36 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> static constexpr size_t size() noexcept { return sizeof...(_Idx); }
> };
>
> +#if __glibcxx_integer_sequence >= 202511L // 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>
> + 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 cfb90533ce4..8db3c1d2df3 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 22dd31b9d32..455d7190e90 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 00000000000..afa644e7dab
> --- /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 00000000000..8640114e7d3
> --- /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 c009062b55c..4cc56c5f76e 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
> --
> 2.53.0
>
>
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