[gcc r14-8170] libstdc++: Implement P2540R1 change to views::cartesian_product()

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:98966f32f906303cd7d2e1c418f320c483e6dcbe

commit r14-8170-g98966f32f906303cd7d2e1c418f320c483e6dcbe
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Jan 16 21:20:12 2024 -0500

    libstdc++: Implement P2540R1 change to views::cartesian_product()
    
    This paper changes the identity element of views::cartesian_product to a
    singleton range instead of an empty range.  It was approved alongside
    the main cartesian_product paper P2374R4, but unfortunately was overlooked
    when implementing the main paper.
    
    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.
    
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

Diff:
---
 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 );