[PATCH] libstdc++: Implement P2540R1 change to views::cartesian_product()

[Patrick Palka]

Tested on x86_64-pc-linux-gnu, does this look OK for trunk/13?

-- >8 --

This paper changes the identity element of views::cartesian_product to
be a single empty tuple instead of an empty range.

libstdc++-v3/ChangeLog:

        * include/std/ranges (views::_CartesianProduct::operator()):
        Adjust identity case as per P2540R1.
        * testsuite/std/ranges/cartesian_product/1.cc (test01):
        Adjust expected result of the identity case.
---
 libstdc++-v3/include/std/ranges                          | 2 +-
 libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 3135e6f0c08..7ef835f486a 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -8748,7 +8748,7 @@ namespace views::__adaptor
        operator() [[nodiscard]] (_Ts&&... __ts) const
        {
          if constexpr (sizeof...(_Ts) == 0)
-           return views::empty<tuple<>>;
+           return views::single(tuple{});
          else
            return 
cartesian_product_view<all_t<_Ts>...>(std::forward<_Ts>(__ts)...);
        }
diff --git a/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc 
b/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
index f69f965c0ce..7281cd8a2f4 100644
--- a/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
+++ b/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
@@ -24,9 +24,9 @@ test01()
   int w[] = {9};
 
   auto v0 = views::cartesian_product();
-  VERIFY( ranges::end(v0) - ranges::begin(v0) == 0 );
-  VERIFY( ranges::size(v0) == 0 );
-  VERIFY( ranges::empty(v0) );
+  VERIFY( ranges::end(v0) - ranges::begin(v0) == 1 );
+  VERIFY( ranges::size(v0) == 1 );
+  VERIFY( !ranges::empty(v0) );
 
   auto v1 = views::cartesian_product(x);
   VERIFY( ranges::end(v1) - ranges::begin(v1) == 3 );
-- 
2.43.0.334.gd4dbce1db5