Re: [PATCH] libstdc++: Implement LWG 3481 change to ranges::viewable_range

[Jonathan Wakely <jwakely@redhat.com>]

On Wed, 20 Oct 2021 at 18:01, Patrick Palka wrote:
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk, and branches
> after a while?

Yes for all.

>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/ranges_base.h (viewable_range): Adjust as per
>         LWG 3481.
>         * testsuite/std/ranges/adaptors/all.cc (test07): New test.
> ---
>  libstdc++-v3/include/bits/ranges_base.h           |  3 ++-
>  libstdc++-v3/testsuite/std/ranges/adaptors/all.cc | 15 +++++++++++++++
>  2 files changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/include/bits/ranges_base.h b/libstdc++-v3/include/bits/ranges_base.h
> index 7801b2fd023..43b0b9f7bf3 100644
> --- a/libstdc++-v3/include/bits/ranges_base.h
> +++ b/libstdc++-v3/include/bits/ranges_base.h
> @@ -688,7 +688,8 @@ namespace ranges
>    /// A range which can be safely converted to a view.
>    template<typename _Tp>
>      concept viewable_range = range<_Tp>
> -      && (borrowed_range<_Tp> || view<remove_cvref_t<_Tp>>);
> +      && ((view<remove_cvref_t<_Tp>> && constructible_from<remove_cvref_t<_Tp>, _Tp>)
> +         || (!view<remove_cvref_t<_Tp>> && borrowed_range<_Tp>));
>
>    // [range.iter.ops] range iterator operations
>
> diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc
> index 9a6a31e6cb4..ce1384c2890 100644
> --- a/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc
> +++ b/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc
> @@ -159,6 +159,20 @@ test06()
>    static_assert(!noexcept(views::all(BorrowedRange<false, false>(x))));
>  }
>
> +void
> +test07()
> +{
> +  // LWG 3481
> +  struct view_t : ranges::empty_view<int> { // move-only view
> +    view_t(const view_t&) = delete;
> +    view_t(view_t&&) = default;
> +    view_t& operator=(const view_t&) = delete;
> +    view_t& operator=(view_t&&) = default;
> +  };
> +  static_assert(std::movable<view_t> && !std::copyable<view_t>);
> +  static_assert(!ranges::viewable_range<view_t&>);
> +}
> +
>  int
>  main()
>  {
> @@ -168,4 +182,5 @@ main()
>    static_assert(test04());
>    test05();
>    test06();
> +  test07();
>  }
> --
> 2.33.1.711.g9d530dc002
>