[PATCH] libstdc++: Implement P1789R3: structured bindings for std::integer_sequence

[Ivan Lazaric]

P1789 enables accessing the sequence values through structured bindings.
```
auto [...values] = std::make_index_sequence<10>{};
// values is a pack of size 10, and elements 0, 1, 2, ..., 9
```

Corresponding C++ draft commit: 3d71a838ed2a1689dd329f964ec4d58152487151
Feature-test macro integer_sequence has been bumped to 202511L.

libstdc++-v3/ChangeLog:

	* include/bits/utility.h:
        Implement structured bindings protocol for std::integer_sequence.
	* include/bits/version.def: Bump integer_sequence feature-test macro.
	* include/bits/version.h: Regenerate.
	* testsuite/20_util/integer_sequence/structured_binding.cc: New test.

Signed-off-by: Ivan Lazaric <ivan.lazaric1@gmail.com>
---
Tested everything under 20_util, including the new test.

 libstdc++-v3/include/bits/utility.h           | 28 ++++++++++++++++++
 libstdc++-v3/include/bits/version.def         |  5 ++++
 libstdc++-v3/include/bits/version.h           |  7 ++++-
 .../integer_sequence/structured_binding.cc    | 29 +++++++++++++++++++
 4 files changed, 68 insertions(+), 1 deletion(-)
 create mode 100644 libstdc++-v3/testsuite/20_util/integer_sequence/structured_binding.cc

diff --git a/libstdc++-v3/include/bits/utility.h b/libstdc++-v3/include/bits/utility.h
index bd6b18d54dd..159884dd814 100644
--- a/libstdc++-v3/include/bits/utility.h
+++ b/libstdc++-v3/include/bits/utility.h
@@ -150,6 +150,34 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static constexpr size_t size() noexcept { return sizeof...(_Idx); }
     };
 
+#if __glibcxx_integer_sequence >= 202511L // C++ >= 26
+    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 b89e60d156d..6103f6a41d1 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 d73b547e1b0..0de2ad9e5f3 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/structured_binding.cc b/libstdc++-v3/testsuite/20_util/integer_sequence/structured_binding.cc
new file mode 100644
index 00000000000..b8e9fa1caa1
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/integer_sequence/structured_binding.cc
@@ -0,0 +1,29 @@
+// { dg-do compile { target c++26 } }
+
+#include <utility>
+
+#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>, size_t>);
+static_assert(std::get<7>(IS1{}) == 7);
+static_assert(destructure_sum(IS1{}) == 45);
+
+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);
+
+using IS3 = std::integer_sequence<char>;
+static_assert(std::tuple_size_v<IS3> == 0);
-- 
2.43.0