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libsdc++: Restore check for validity of std::get for elements_view.

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Resolves LWG3797, "elements_view insufficiently constrained".

When P2165R4 updated __has_tuple_element in C++23 to reuse __tuple_like
concept, it dropped the requirement of validity of get, assuming that for
tuple_like type with size of N, get<I> on lvalue is well-formed for any I < N.
This however does not hold for ranges::subrange (tuple-like of size 2) with
move-only iterator, for which get can only be applied on rvalue. In consequence
constrains allowed instantiating elements_view for range of such subrange,
but instantiating it's iterator lead to hard error from iterator_category
computation.

This patch applies the requirements on validity of get also in C++23 and
later standard modes.

libstdc++-v3/ChangeLog:

	* include/std/ranges (__detail::__has_tuple_element): Check
	if std::get<_Nm>(__t) returns referenceable type also for C++23
	and later.
	* testsuite/std/ranges/adaptors/elements.cc: Add test covering
	vector of ranges::subrange with move-only iterator.

Reviewed-by: Patrick Palka <ppalka@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
This commit is contained in:
Tomasz Kamiński
2026-04-24 15:10:41 +02:00
parent 3f6d37193c
commit 490eca9b76
2 changed files with 24 additions and 1 deletions
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9
libstdc++-v3/include/std/ranges
View File

@@ -4355,8 +4355,15 @@ namespace views::__adaptor
namespace __detail
{
#if __cpp_lib_tuple_like // >= C++23
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 3797. elements_view insufficiently constrained
template<typename _Tp, size_t _Nm>
concept __has_tuple_element = __tuple_like<_Tp> && _Nm < tuple_size_v<_Tp>;
concept __has_tuple_element = __tuple_like<_Tp> && _Nm < tuple_size_v<_Tp>
&& requires(_Tp __t)
{
{ std::get<_Nm>(__t) }
-> convertible_to<const tuple_element_t<_Nm, _Tp>&>;
};
#else
template<typename _Tp, size_t _Nm>
concept __has_tuple_element = requires(_Tp __t)

16
libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc
View File

@@ -27,6 +27,22 @@
namespace ranges = std::ranges;
namespace views = ranges::views;
template<size_t N, typename Rg>
concept can_elements = requires (Rg&& rg)
{ views::elements<0>(rg); };
static_assert( !can_elements<0, std::vector<int>> );
static_assert( can_elements<0, std::tuple<int>[4]> );
static_assert( can_elements<1, std::vector<std::pair<int, int>>> );
// Test LWG 3797. elements_view insufficiently constrained
using move_only_iter_range = __gnu_test::test_input_range_nocopy<int>;
using move_only_iter_subrange = ranges::subrange<
ranges::iterator_t<move_only_iter_range>,
ranges::sentinel_t<move_only_iter_range>>;
static_assert( can_elements<0, std::vector<ranges::subrange<int*>>> );
static_assert( !can_elements<0, std::vector<move_only_iter_subrange>> );
void
test01()
{