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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 98966f32f9)
This commit is contained in:
Patrick Palka 2024-01-16 21:20:12 -05:00
parent d6551d6d83
commit d28b05bf57
2 changed files with 4 additions and 4 deletions
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2
libstdc++-v3/include/std/ranges
View File

@ -8549,7 +8549,7 @@ namespace views::__adaptor
operator() [[nodiscard]] (_Ts&&... __ts) const operator() [[nodiscard]] (_Ts&&... __ts) const
{ {
if constexpr (sizeof...(_Ts) == 0) if constexpr (sizeof...(_Ts) == 0)
return views::empty<tuple<>>; return views::single(tuple{});
else else
return cartesian_product_view<all_t<_Ts>...>(std::forward<_Ts>(__ts)...); return cartesian_product_view<all_t<_Ts>...>(std::forward<_Ts>(__ts)...);
} }

6
libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
View File

@ -23,9 +23,9 @@ test01()
int w[] = {9}; int w[] = {9};
auto v0 = views::cartesian_product(); auto v0 = views::cartesian_product();
VERIFY( ranges::end(v0) - ranges::begin(v0) == 0 ); VERIFY( ranges::end(v0) - ranges::begin(v0) == 1 );
VERIFY( ranges::size(v0) == 0 ); VERIFY( ranges::size(v0) == 1 );
VERIFY( ranges::empty(v0) ); VERIFY( !ranges::empty(v0) );
auto v1 = views::cartesian_product(x); auto v1 = views::cartesian_product(x);
VERIFY( ranges::end(v1) - ranges::begin(v1) == 3 ); VERIFY( ranges::end(v1) - ranges::begin(v1) == 3 );