libstdc++: Implement P3138R5 views::cache_latest

libstdc++-v3/ChangeLog:

	* include/bits/version.def (ranges_cache_latest): Define.
	* include/bits/version.h: Regenerate.
	* include/std/ranges (__detail::__non_propagating_cache::_M_reset):
	Export from base class _Optional_base.
	(cache_latest_view): Define for C++26.
	(cache_latest_view::_Iterator): Likewise.
	(cache_latest_view::_Sentinel): Likewise.
	(views::__detail::__can_cache_latest): Likewise.
	(views::_CacheLatest, views::cache_latest): Likewise.
	* testsuite/std/ranges/adaptors/cache_latest/1.cc: New test.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

1 files changed, 72 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/cache_latest/1.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/cache_latest/1.cc
new file mode 100644
index 0000000..5904831
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/cache_latest/1.cc
@@ -0,0 +1,72 @@
+// { dg-do run { target c++26 } }
+
+#include <ranges>
+
+#if __cpp_lib_ranges_cache_latest != 202411L
+# error "Feature-test macro __cpp_lib_ranges_cache_latest has wrong value in <ranges>"
+#endif
+
+#include <algorithm>
+#include <testsuite_hooks.h>
+
+namespace ranges = std::ranges;
+namespace views = std::views;
+
+constexpr bool
+test01()
+{
+  int xs[] = {1,2,3,4,5};
+  auto v = xs | views::cache_latest;
+  VERIFY( ranges::equal(v, xs) );
+  VERIFY( ranges::size(v) == 5 );
+
+  auto it = v.begin();
+  auto st = v.end();
+  VERIFY( st - it == 5 );
+  VERIFY( it - st == -5 );
+  it++;
+  VERIFY( st - it == 4 );
+  VERIFY( it - st == -4 );
+
+  auto jt = v.begin();
+  ranges::iter_swap(it, jt);
+  VERIFY( ranges::equal(xs, (int[]){2,1,3,4,5}) );
+  int n = ranges::iter_move(it);
+  VERIFY( n == 1 );
+  ranges::iter_swap(it, jt);
+
+  auto w = views::iota(1, 6) | views::cache_latest;
+  VERIFY( ranges::equal(w, xs) );
+
+  return true;
+}
+
+constexpr bool
+test02()
+{
+  // Motivating example from P3138R5
+  int xs[] = {1, 2, 3, 4, 5};
+  int transform_count = 0;
+  auto v = xs | views::transform([&](int i){ ++transform_count; return i * i; })
+	      | views::filter([](int i){ return i % 2 == 0; });
+  VERIFY( ranges::equal(v, (int[]){4, 16}) );
+  VERIFY( transform_count == 7 );
+
+  transform_count = 0;
+  auto w = xs | views::transform([&](int i){ ++transform_count; return i * i; })
+	      | views::cache_latest
+	      | views::filter([](int i){ return i % 2 == 0; });
+  VERIFY( ranges::equal(w, (int[]){4, 16}) );
+  VERIFY( transform_count == 5 );
+
+  return true;
+}
+
+int
+main()
+{
+  static_assert(test01());
+  static_assert(test02());
+  test01();
+  test02();
+}