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>
(cherry picked from commit 98966f32f906303cd7d2e1c418f320c483e6dcbe)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index daf3f32b808ce3910a8d3ce6e7cbda020f5aea2a..dda61349c8fdf46c4920b74e6b5e440b2c0228a4 100644 (file)
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -8549,7 +8549,7 @@ 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 ef2ece4a168608f194f9720acf320082e0d817a1..a44465fc7e55ef6810c979fe653c0f688208c3a9 100644 (file)
--- a/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
+++ b/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
@@ -23,9 +23,9 @@ 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 );