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libstdc++: Reject using views::iota on iota_view.

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Resolves LWG4096, views::iota(views::iota(0)) should be rejected.

For __e of type _Tp that is specialization of iota_view, the CTAD based
expression iota_view(__e) is well formed, and creates a copy of __e.
As iota_view<decay_t<_Tp>> is ill-formed in this case (iota_view is not
weakly_incrementable), using that type in return type explicitly, removes
the overload from overload resolution in this case.

The (now redudant) __detail::__can_iota_view constrain in template head is
preserved, to provide error messages consistent with adaptors for other
non-incrementable types.

libstdc++-v3/ChangeLog:

	* include/std/ranges (_Iota::operator()(_Tp&&)): Replace
	auto return type and CTAD with iota_view<decay_t<_Tp>>.
	* testsuite/std/ranges/iota/iota_view.cc: Tests if
	views::iota(iota_view) is rejected.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
This commit is contained in:
Tomasz Kamiński
2026-04-24 13:02:22 +02:00
parent 8defa4b8b1
commit 9a00a26364
2 changed files with 15 additions and 2 deletions
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6
libstdc++-v3/include/std/ranges
View File

@@ -900,10 +900,12 @@ namespace views
struct _Iota
{
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 4096. views::iota(views::iota(0)) should be rejected
template<__detail::__can_iota_view _Tp>
constexpr auto
constexpr iota_view<decay_t<_Tp>>
operator() [[nodiscard]] (_Tp&& __e) const
{ return iota_view(std::forward<_Tp>(__e)); }
{ return iota_view<decay_t<_Tp>>(std::forward<_Tp>(__e)); }
template<typename _Tp, typename _Up>
requires __detail::__can_iota_view<_Tp, _Up>

11
libstdc++-v3/testsuite/std/ranges/iota/iota_view.cc
View File

@@ -22,6 +22,17 @@
#include <vector>
#include <testsuite_hooks.h>
template<typename Inc>
concept can_iota = requires (Inc&& __inc)
{ std::views::iota(__inc); };
static_assert( can_iota<int> );
static_assert( can_iota<int*> );
static_assert( !can_iota<std::ranges::iota_view<int>> );
static_assert( !can_iota<const std::ranges::iota_view<int>> );
static_assert( !can_iota<std::ranges::iota_view<int>&> );
static_assert( !can_iota<const std::ranges::iota_view<int>&> );
void
test01()
{