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libstdc++: Update LWG 4166 changes to concat_view::end() [PR120934]

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In r15-4555-gf191c830154565 we proactively implemented the initial
proposed resolution for LWG 4166 which later turned out to be
insufficient, since we must also require equality_comparable of the
underlying iterators before concat_view could be a common range.

This patch implements the updated P/R, requiring all underlying
iterators to be forward (which implies equality_comparable) before
making concat_view common, which fixes the testcase from this PR.

	PR libstdc++/120934

libstdc++-v3/ChangeLog:

	* include/std/ranges (concat_view::end): Refine condition
	for returning an iterator instead of default_sentinel as
	per the updated P/R for LWG 4166.
	* testsuite/std/ranges/concat/1.cc (test05): New test.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
This commit is contained in:
Patrick Palka
2025-07-03 10:55:17 -04:00
parent 13c766066e
commit c5a17e92eb
2 changed files with 15 additions and 2 deletions
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4
libstdc++-v3/include/std/ranges
View File

@@ -9735,7 +9735,7 @@ namespace ranges
end() requires (!(__detail::__simple_view<_Vs> && ...))
{
constexpr auto __n = sizeof...(_Vs);
if constexpr ((semiregular<iterator_t<_Vs>> && ...)
if constexpr (__detail::__all_forward<false, _Vs...>
&& common_range<_Vs...[__n - 1]>)
return _Iterator<false>(this, in_place_index<__n - 1>,
ranges::end(std::get<__n - 1>(_M_views)));
@@ -9747,7 +9747,7 @@ namespace ranges
end() const requires (range<const _Vs> && ...) && __detail::__concatable<const _Vs...>
{
constexpr auto __n = sizeof...(_Vs);
if constexpr ((semiregular<iterator_t<const _Vs>> && ...)
if constexpr (__detail::__all_forward<true, _Vs...>
&& common_range<const _Vs...[__n - 1]>)
return _Iterator<true>(this, in_place_index<__n - 1>,
ranges::end(std::get<__n - 1>(_M_views)));

13
libstdc++-v3/testsuite/std/ranges/concat/1.cc
View File

@@ -99,6 +99,18 @@ test04()
using type = decltype(v);
}
void
test05()
{
// PR libstdc++/120934 - views::concat is ill-formed depending on argument order
auto v1 = views::single(1);
std::vector<int> vec = {2, 3};
auto v2 = views::join(views::transform(vec, views::single));
static_assert( ranges::range<decltype(views::concat(v1, v2))> );
static_assert( ranges::range<decltype(views::concat(v2, v1))> );
}
int
main()
{
@@ -107,4 +119,5 @@ main()
test02();
test03();
test04();
test05();
}