[gcc r16-3911] libstdc++: Fix missing change to views::pairwise from P2165R4 [PR121956]

[Jonathan Wakely]

https://gcc.gnu.org/g:0c762f79a9e7bbd8d8fc5e639d53e43d9e331299

commit r16-3911-g0c762f79a9e7bbd8d8fc5e639d53e43d9e331299
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Sep 16 13:41:20 2025 +0100

    libstdc++: Fix missing change to views::pairwise from P2165R4 [PR121956]
    
    ranges::adjacent_view::_Iterator::value_type should have been changed by
    r14-8710-g65b4cba9d6a9ff to always produce std::tuple, even for the
    N == 2 views::pairwise specialization.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/121956
            * include/std/ranges (adjacent_view::_Iterator::value_type):
            Always define as std::tuple<T, N>, not std::pair<T, T>.
            * testsuite/std/ranges/adaptors/adjacent/1.cc: Check value type
            of views::pairwise.
    
    Reviewed-by: Patrick Palka <ppalka@redhat.com>

Diff:
---
 libstdc++-v3/include/std/ranges                          |  4 +---
 libstdc++-v3/testsuite/std/ranges/adaptors/adjacent/1.cc | 13 +++++++++++++
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index f493da56203b..1c76dcc85acd 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -5460,9 +5460,7 @@ namespace views::__adaptor
   public:
     using iterator_category = input_iterator_tag;
     using iterator_concept = decltype(_S_iter_concept());
-    using value_type = conditional_t<_Nm == 2,
-				     pair<range_value_t<_Base>, range_value_t<_Base>>,
-				     __detail::__repeated_tuple<range_value_t<_Base>, _Nm>>;
+    using value_type = __detail::__repeated_tuple<range_value_t<_Base>, _Nm>;
     using difference_type = range_difference_t<_Base>;
 
     _Iterator() = default;
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent/1.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent/1.cc
index 085cd4a8c54c..0a5c67f56140 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent/1.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent/1.cc
@@ -114,6 +114,18 @@ test04()
   return true;
 }
 
+constexpr bool
+test05()
+{
+  // PR libstdc++/121956
+  int a[2]{};
+  __gnu_test::test_random_access_range r(a);
+  auto v = r | views::pairwise;
+  static_assert( std::is_same_v<ranges::range_value_t<decltype(v)>,
+				std::tuple<int, int>> );
+  return true;
+}
+
 int
 main()
 {
@@ -121,4 +133,5 @@ main()
   static_assert(test02());
   static_assert(test03());
   static_assert(test04());
+  static_assert(test05());
 }