On Wed, 12 Mar 2025, Patrick Palka wrote:
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk/14 and
> perhaps 13?
>
> N.B. the use of a constrained auto instead of a separate static_assert
> in the testcase is unfortunate but I opted for local consistency for
> now.
>
> -- >8 --
>
> Unlike for span<X> and empty_view<X>, the range_reference_t of
> ref_view<X> doesn't correspond to X. This patch fixes the ref_view
> branch of views::as_const to correctly query the underlying range
> type X.
>
> PR libstdc++/119135
>
> libstdc++-v3/ChangeLog:
>
> * include/std/ranges: Include <utility>.
> (views::__detail::__is_ref_view): Replace with ...
> (views::__detail::__is_constable_ref_view): ... this.
> (views::_AsConst::operator()): Correct the ref_view branch.
> * testsuite/std/ranges/adaptors/as_const/1.cc (test03): Extend
> test.
> ---
> libstdc++-v3/include/std/ranges | 12 ++++++------
> .../testsuite/std/ranges/adaptors/as_const/1.cc | 4 ++++
> 2 files changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> index c2a2d6f4e05..31d62454895 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -48,6 +48,7 @@
> #include <string_view>
> #include <tuple>
> #if __cplusplus > 202002L
> +#include <utility>
> #include <variant>
> #endif
> #include <bits/ranges_util.h>
> @@ -9324,10 +9325,11 @@ namespace views::__adaptor
> namespace __detail
> {
> template<typename _Tp>
> - inline constexpr bool __is_ref_view = false;
> + inline constexpr bool __is_constable_ref_view = false;
>
> template<typename _Range>
> - inline constexpr bool __is_ref_view<ref_view<_Range>> = true;
> + inline constexpr bool __is_constable_ref_view<ref_view<_Range>>
> + = constant_range<const _Range>;
>
> template<typename _Range>
> concept __can_as_const_view = requires {
> as_const_view(std::declval<_Range>()); };
> @@ -9349,10 +9351,8 @@ namespace views::__adaptor
> return views::empty<const element_type>;
> else if constexpr (std::__detail::__is_span<_Tp>)
> return span<const element_type,
> _Tp::extent>(std::forward<_Range>(__r));
> - else if constexpr (__detail::__is_ref_view<_Tp>
> - && constant_range<const element_type>)
> - return ref_view(static_cast<const element_type&>
> - (std::forward<_Range>(__r).base()));
> + else if constexpr (__detail::__is_constable_ref_view<_Tp>)
> + return ref_view(std::as_const(__r.base()));
Whoops, just noticed that I got rid of the perfect forwarding of __r
here for no good reason. It shouldn't matter since its base() member
function is const, but consider the std::forward restored for
consistency.
> else if constexpr (is_lvalue_reference_v<_Range>
> && constant_range<const _Tp>
> && !view<_Tp>)
> diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/as_const/1.cc
> b/libstdc++-v3/testsuite/std/ranges/adaptors/as_const/1.cc
> index c36786a8c5f..3f1f8eb1772 100644
> --- a/libstdc++-v3/testsuite/std/ranges/adaptors/as_const/1.cc
> +++ b/libstdc++-v3/testsuite/std/ranges/adaptors/as_const/1.cc
> @@ -63,6 +63,10 @@ test03()
> std::vector<int> v;
> std::same_as<ranges::ref_view<const std::vector<int>>>
> auto r = views::as_const(v);
> +
> + // PR libstdc++/119135
> + std::same_as<ranges::ref_view<const std::vector<int>>>
> + auto r2 = views::as_const(views::all(v));
> }
>
> int
> --
> 2.49.0.rc1.37.ge969bc8759
>
>
