[{"id":3673552,"web_url":"http://patchwork.ozlabs.org/comment/3673552/","msgid":"","list_archive_url":null,"date":"2026-04-05T09:46:01","subject":"Re: [PATCH] libstdc++: Implement P1789R3: structured bindings for\n std::integer_sequence","submitter":{"id":90409,"url":"http://patchwork.ozlabs.org/api/people/90409/","name":"Tomasz Kaminski","email":"tkaminsk@redhat.com"},"content":"On Sat, Apr 4, 2026 at 7:03 PM Ivan Lazaric wrote:\n\n> P1789 enables accessing the sequence values through structured bindings.\n> ```\n> auto [...values] = std::make_index_sequence<10>{};\n> // values is a pack of size 10, and elements 0, 1, 2, ..., 9\n> ```\n>\n> Corresponding C++ draft commit: 3d71a838ed2a1689dd329f964ec4d58152487151\n> Feature-test macro integer_sequence has been bumped to 202511L.\n>\n> libstdc++-v3/ChangeLog:\n>\n> * include/bits/utility.h:\n> Implement structured bindings protocol for std::integer_sequence.\n> * include/bits/version.def: Bump integer_sequence feature-test\n> macro.\n> * include/bits/version.h: Regenerate.\n> * testsuite/20_util/integer_sequence/structured_binding.cc: New\n> test.\n>\n> Signed-off-by: Ivan Lazaric \n> ---\n> Tested everything under 20_util, including the new test.\n>\n> libstdc++-v3/include/bits/utility.h | 28 ++++++++++++++++++\n> libstdc++-v3/include/bits/version.def | 5 ++++\n> libstdc++-v3/include/bits/version.h | 7 ++++-\n> .../integer_sequence/structured_binding.cc | 29 +++++++++++++++++++\n> 4 files changed, 68 insertions(+), 1 deletion(-)\n> create mode 100644\n> libstdc++-v3/testsuite/20_util/integer_sequence/structured_binding.cc\n>\n> diff --git a/libstdc++-v3/include/bits/utility.h\n> b/libstdc++-v3/include/bits/utility.h\n> index bd6b18d54dd..159884dd814 100644\n> --- a/libstdc++-v3/include/bits/utility.h\n> +++ b/libstdc++-v3/include/bits/utility.h\n> @@ -150,6 +150,34 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n> static constexpr size_t size() noexcept { return sizeof...(_Idx); }\n> };\n>\n> +#if __glibcxx_integer_sequence >= 202511L // C++ >= 26\n> + template\n> + struct tuple_size>\n> + : integral_constant { };\n> +\n> + template\n> + struct tuple_element<__i, integer_sequence<_Tp, _Idx...>>\n> + {\n> + static_assert(__i < sizeof...(_Idx));\n> + using type = _Tp;\n> + };\n> +\n> + template\n> + struct tuple_element<__i, const integer_sequence<_Tp, _Idx...>>\n> + {\n> + static_assert(__i < sizeof...(_Idx));\n> + using type = _Tp;\n> + };\n> +\n> + template\n> + constexpr _Tp\n> + get (integer_sequence<_Tp, _Idx...>) noexcept\n> + {\n> + static_assert(__i < sizeof...(_Idx));\n> + return _Idx...[__i];\n> + }\n> +#endif // __glibcxx_integer_sequence >= 202511L\n> +\n> /// Alias template make_integer_sequence\n> template\n> using make_integer_sequence\n> diff --git a/libstdc++-v3/include/bits/version.def\n> b/libstdc++-v3/include/bits/version.def\n> index b89e60d156d..6103f6a41d1 100644\n> --- a/libstdc++-v3/include/bits/version.def\n> +++ b/libstdc++-v3/include/bits/version.def\n> @@ -184,6 +184,11 @@ ftms = {\n>\n> ftms = {\n> name = integer_sequence;\n> + values = {\n> + v = 202511;\n> + cxxmin = 26;\n> + extra_cond = \"__cpp_pack_indexing\";\n> + };\n> values = {\n> v = 201304;\n> cxxmin = 14;\n> diff --git a/libstdc++-v3/include/bits/version.h\n> b/libstdc++-v3/include/bits/version.h\n> index d73b547e1b0..0de2ad9e5f3 100644\n> --- a/libstdc++-v3/include/bits/version.h\n> +++ b/libstdc++-v3/include/bits/version.h\n> @@ -186,7 +186,12 @@\n> #undef __glibcxx_want_exchange_function\n>\n> #if !defined(__cpp_lib_integer_sequence)\n> -# if (__cplusplus >= 201402L)\n> +# if (__cplusplus > 202302L) && (__cpp_pack_indexing)\n> +# define __glibcxx_integer_sequence 202511L\n> +# if defined(__glibcxx_want_all) ||\n> defined(__glibcxx_want_integer_sequence)\n> +# define __cpp_lib_integer_sequence 202511L\n> +# endif\n> +# elif (__cplusplus >= 201402L)\n> # define __glibcxx_integer_sequence 201304L\n> # if defined(__glibcxx_want_all) ||\n> defined(__glibcxx_want_integer_sequence)\n> # define __cpp_lib_integer_sequence 201304L\n> diff --git\n> a/libstdc++-v3/testsuite/20_util/integer_sequence/structured_binding.cc\n> b/libstdc++-v3/testsuite/20_util/integer_sequence/structured_binding.cc\n> new file mode 100644\n> index 00000000000..b8e9fa1caa1\n> --- /dev/null\n> +++ b/libstdc++-v3/testsuite/20_util/integer_sequence/structured_binding.cc\n> @@ -0,0 +1,29 @@\n> +// { dg-do compile { target c++26 } }\n> +\n> +#include \n> +\n> +#if __cpp_lib_integer_sequence < 202511L\n> +# error \"Feature-test macro __cpp_lib_integer_sequence is incorrect\"\n> +#endif\n> +\n> +constexpr auto\n> +destructure_sum(auto seq)\n> +{\n> + auto [...elems] = seq;\n\nThere is already patch for this feature from Matthias Wippic:\nhttps://gcc.gnu.org/pipermail/libstdc++/2025-November/064271.html\n\nHowever, it got stuck because before the Croydon meeting, as currently\nspecified the following is ill-formed:\n auto [...elems] == std::make_integer_sequence<10>{};\n(Could you add test like above):\n\nThis was addressed by CWG3135 (\nhttps://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#3135)\nthat was accepted in Croydon, but I do not think we should ship that\nfeature before it is resolved on trunk.\n\n\n+ return (0 + ... + elems);\n\n+}\n> +\n> +using IS1 = std::make_index_sequence<10>;\n> +static_assert(std::tuple_size_v == 10);\n> +static_assert(std::is_same_v, size_t>);\n> +static_assert(std::get<7>(IS1{}) == 7);\n> +static_assert(destructure_sum(IS1{}) == 45);\n> +\n> +using IS2 = std::integer_sequence;\n> +static_assert(std::tuple_size_v == 3);\n> +static_assert(std::is_same_v, int>);\n> +static_assert(std::get<2>(IS2{}) == -13);\n> +static_assert(destructure_sum(IS2{}) == 130);\n> +\n> +using IS3 = std::integer_sequence;\n> +static_assert(std::tuple_size_v == 0);\n> --\n> 2.43.0\n>\n>","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=PfobcUy1;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=2620:52:6:3111::32; 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Sun, 05 Apr 2026\n 02:46:12 -0700 (PDT)"],"MIME-Version":"1.0","References":"<20260404170122.2928689-2-ivan.lazaric1@gmail.com>","In-Reply-To":"<20260404170122.2928689-2-ivan.lazaric1@gmail.com>","From":"Tomasz Kaminski ","Date":"Sun, 5 Apr 2026 11:46:01 +0200","X-Gm-Features":"AQROBzB46jz7wcHm35itx1NupEGhkSUNK2gK0vUq5JOP6I56dBfxNjoZwO8dzc0","Message-ID":"\n ","Subject":"Re: [PATCH] libstdc++: Implement P1789R3: structured bindings for\n std::integer_sequence","To":"Ivan Lazaric ","Cc":"libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org","X-Mimecast-Spam-Score":"0","X-Mimecast-MFC-PROC-ID":"SIFvPVpF0khByPidNTfFzQzS0PL0GxkxpydlDE_2w14_1775382373","X-Mimecast-Originator":"redhat.com","Content-Type":"multipart/alternative; boundary=\"000000000000a64385064eb36b92\"","X-BeenThere":"gcc-patches@gcc.gnu.org","X-Mailman-Version":"2.1.30","Precedence":"list","List-Id":"Gcc-patches mailing list ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Errors-To":"gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org"}},{"id":3673562,"web_url":"http://patchwork.ozlabs.org/comment/3673562/","msgid":"","list_archive_url":null,"date":"2026-04-05T12:11:07","subject":"Re: [PATCH] libstdc++: Implement P1789R3: structured bindings for\n std::integer_sequence","submitter":{"id":92628,"url":"http://patchwork.ozlabs.org/api/people/92628/","name":"Ivan Lazaric","email":"ivan.lazaric1@gmail.com"},"content":"Will post an updated patch, but wouldn’t pre-CWG3135 wording also work, due\nto lifetime extension?\n\nauto [a, b, c] = std::make_index_sequence<3>{};// as-ifauto seq =\nstd::make_index_sequence<3>{};\nstd::size_t&& a = std::get<0>(seq);\nstd::size_t&& b = std::get<1>(seq);\nstd::size_t&& c = std::get<2>(seq);\n\n\nOn Sun, Apr 5, 2026 at 11:46 AM Tomasz Kaminski wrote:\n\n>\n>\n> On Sat, Apr 4, 2026 at 7:03 PM Ivan Lazaric \n> wrote:\n>\n>> P1789 enables accessing the sequence values through structured bindings.\n>> ```\n>> auto [...values] = std::make_index_sequence<10>{};\n>> // values is a pack of size 10, and elements 0, 1, 2, ..., 9\n>> ```\n>>\n>> Corresponding C++ draft commit: 3d71a838ed2a1689dd329f964ec4d58152487151\n>> Feature-test macro integer_sequence has been bumped to 202511L.\n>>\n>> libstdc++-v3/ChangeLog:\n>>\n>> * include/bits/utility.h:\n>> Implement structured bindings protocol for std::integer_sequence.\n>> * include/bits/version.def: Bump integer_sequence feature-test\n>> macro.\n>> * include/bits/version.h: Regenerate.\n>> * testsuite/20_util/integer_sequence/structured_binding.cc: New\n>> test.\n>>\n>> Signed-off-by: Ivan Lazaric \n>> ---\n>> Tested everything under 20_util, including the new test.\n>>\n>> libstdc++-v3/include/bits/utility.h | 28 ++++++++++++++++++\n>> libstdc++-v3/include/bits/version.def | 5 ++++\n>> libstdc++-v3/include/bits/version.h | 7 ++++-\n>> .../integer_sequence/structured_binding.cc | 29 +++++++++++++++++++\n>> 4 files changed, 68 insertions(+), 1 deletion(-)\n>> create mode 100644\n>> libstdc++-v3/testsuite/20_util/integer_sequence/structured_binding.cc\n>>\n>> diff --git a/libstdc++-v3/include/bits/utility.h\n>> b/libstdc++-v3/include/bits/utility.h\n>> index bd6b18d54dd..159884dd814 100644\n>> --- a/libstdc++-v3/include/bits/utility.h\n>> +++ b/libstdc++-v3/include/bits/utility.h\n>> @@ -150,6 +150,34 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n>> static constexpr size_t size() noexcept { return sizeof...(_Idx); }\n>> };\n>>\n>> +#if __glibcxx_integer_sequence >= 202511L // C++ >= 26\n>> + template\n>> + struct tuple_size>\n>> + : integral_constant { };\n>> +\n>> + template\n>> + struct tuple_element<__i, integer_sequence<_Tp, _Idx...>>\n>> + {\n>> + static_assert(__i < sizeof...(_Idx));\n>> + using type = _Tp;\n>> + };\n>> +\n>> + template\n>> + struct tuple_element<__i, const integer_sequence<_Tp, _Idx...>>\n>> + {\n>> + static_assert(__i < sizeof...(_Idx));\n>> + using type = _Tp;\n>> + };\n>> +\n>> + template\n>> + constexpr _Tp\n>> + get (integer_sequence<_Tp, _Idx...>) noexcept\n>> + {\n>> + static_assert(__i < sizeof...(_Idx));\n>> + return _Idx...[__i];\n>> + }\n>> +#endif // __glibcxx_integer_sequence >= 202511L\n>> +\n>> /// Alias template make_integer_sequence\n>> template\n>> using make_integer_sequence\n>> diff --git a/libstdc++-v3/include/bits/version.def\n>> b/libstdc++-v3/include/bits/version.def\n>> index b89e60d156d..6103f6a41d1 100644\n>> --- a/libstdc++-v3/include/bits/version.def\n>> +++ b/libstdc++-v3/include/bits/version.def\n>> @@ -184,6 +184,11 @@ ftms = {\n>>\n>> ftms = {\n>> name = integer_sequence;\n>> + values = {\n>> + v = 202511;\n>> + cxxmin = 26;\n>> + extra_cond = \"__cpp_pack_indexing\";\n>> + };\n>> values = {\n>> v = 201304;\n>> cxxmin = 14;\n>> diff --git a/libstdc++-v3/include/bits/version.h\n>> b/libstdc++-v3/include/bits/version.h\n>> index d73b547e1b0..0de2ad9e5f3 100644\n>> --- a/libstdc++-v3/include/bits/version.h\n>> +++ b/libstdc++-v3/include/bits/version.h\n>> @@ -186,7 +186,12 @@\n>> #undef __glibcxx_want_exchange_function\n>>\n>> #if !defined(__cpp_lib_integer_sequence)\n>> -# if (__cplusplus >= 201402L)\n>> +# if (__cplusplus > 202302L) && (__cpp_pack_indexing)\n>> +# define __glibcxx_integer_sequence 202511L\n>> +# if defined(__glibcxx_want_all) ||\n>> defined(__glibcxx_want_integer_sequence)\n>> +# define __cpp_lib_integer_sequence 202511L\n>> +# endif\n>> +# elif (__cplusplus >= 201402L)\n>> # define __glibcxx_integer_sequence 201304L\n>> # if defined(__glibcxx_want_all) ||\n>> defined(__glibcxx_want_integer_sequence)\n>> # define __cpp_lib_integer_sequence 201304L\n>> diff --git\n>> a/libstdc++-v3/testsuite/20_util/integer_sequence/structured_binding.cc\n>> b/libstdc++-v3/testsuite/20_util/integer_sequence/structured_binding.cc\n>> new file mode 100644\n>> index 00000000000..b8e9fa1caa1\n>> --- /dev/null\n>> +++\n>> b/libstdc++-v3/testsuite/20_util/integer_sequence/structured_binding.cc\n>> @@ -0,0 +1,29 @@\n>> +// { dg-do compile { target c++26 } }\n>> +\n>> +#include \n>> +\n>> +#if __cpp_lib_integer_sequence < 202511L\n>> +# error \"Feature-test macro __cpp_lib_integer_sequence is incorrect\"\n>> +#endif\n>> +\n>> +constexpr auto\n>> +destructure_sum(auto seq)\n>> +{\n>> + auto [...elems] = seq;\n>\n> There is already patch for this feature from Matthias Wippic:\n> https://gcc.gnu.org/pipermail/libstdc++/2025-November/064271.html\n>\n> However, it got stuck because before the Croydon meeting, as currently\n> specified the following is ill-formed:\n> auto [...elems] == std::make_integer_sequence<10>{};\n> (Could you add test like above):\n>\n> This was addressed by CWG3135 (\n> https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#3135)\n> that was accepted in Croydon, but I do not think we should ship that\n> feature before it is resolved on trunk.\n>\n>\n> + return (0 + ... + elems);\n>\n> +}\n>> +\n>> +using IS1 = std::make_index_sequence<10>;\n>> +static_assert(std::tuple_size_v == 10);\n>> +static_assert(std::is_same_v, size_t>);\n>> +static_assert(std::get<7>(IS1{}) == 7);\n>> +static_assert(destructure_sum(IS1{}) == 45);\n>> +\n>> +using IS2 = std::integer_sequence;\n>> +static_assert(std::tuple_size_v == 3);\n>> +static_assert(std::is_same_v, int>);\n>> +static_assert(std::get<2>(IS2{}) == -13);\n>> +static_assert(destructure_sum(IS2{}) == 130);\n>> +\n>> +using IS3 = std::integer_sequence;\n>> +static_assert(std::tuple_size_v == 0);\n>> --\n>> 2.43.0\n>>\n>>","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=LrNlcuAT;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=38.145.34.32; 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