[gcc r15-11190] libstdc++/ranges: Implement LWG 3599 change to lazy_split_view

[Patrick Palka]

https://gcc.gnu.org/g:4b8e6ab3a95a871f6ff155913e750ce11f8bf587

commit r15-11190-g4b8e6ab3a95a871f6ff155913e750ce11f8bf587
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Apr 10 10:14:47 2026 -0400

    libstdc++/ranges: Implement LWG 3599 change to lazy_split_view
    
    libstdc++-v3/ChangeLog:
    
            * include/std/ranges (lazy_split_view::begin): Add
            const _Pattern constraint on const overload as per LWG 3599.
            (lazy_split_view::end): Likewise.
            * testsuite/std/ranges/adaptors/lazy_split.cc (test14): New
            test.
    
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
    (cherry picked from commit 8c46c7d647bf42aafab17f7e3dd4c2e18722241f)

Diff:
---
 libstdc++-v3/include/std/ranges                          |  7 ++++++-
 libstdc++-v3/testsuite/std/ranges/adaptors/lazy_split.cc | 10 ++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 5954aa1b54b5..569b681b9fe1 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -3681,8 +3681,12 @@ namespace views::__adaptor
 	  }
       }
 
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3599. The const overload of lazy_split_view::begin should be
+      // constrained by const Pattern
       constexpr auto
       begin() const requires forward_range<_Vp> && forward_range<const _Vp>
+	&& forward_range<const _Pattern>
       {
 	return _OuterIter<true>{this, ranges::begin(_M_base)};
       }
@@ -3700,7 +3704,8 @@ namespace views::__adaptor
       {
 	if constexpr (forward_range<_Vp>
 		      && forward_range<const _Vp>
-		      && common_range<const _Vp>)
+		      && common_range<const _Vp>
+		      && forward_range<const _Pattern>)
 	  return _OuterIter<true>{this, ranges::end(_M_base)};
 	else
 	  return default_sentinel;
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/lazy_split.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/lazy_split.cc
index 321ae271bf2b..498b6f3b0bdf 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/lazy_split.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/lazy_split.cc
@@ -239,6 +239,15 @@ test13()
   constexpr decltype(views::lazy_split(views::single(0), 0).begin()) it;
 }
 
+void
+test14()
+{
+  // LWG 3599
+  auto p = views::iota(0) | views::take(1) | views::reverse;
+  auto r = views::single(42) | views::lazy_split(p);
+  auto f = r.front();
+}
+
 int
 main()
 {
@@ -255,4 +264,5 @@ main()
   test11();
   static_assert(test12());
   test13();
+  test14();
 }