Re: [PATCH] libstdc++: Implement LWG 3820/3849 changes to cartesian_product_view

[Jonathan Wakely <jwakely@redhat.com>]

On Wed, 8 Mar 2023 at 15:13, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> The LWG 3820 testcase revealed a bug in _M_advance, which this patch
> also fixes.
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

OK

>
> libstdc++-v3/ChangeLog:
>
>         * include/std/ranges
>         (cartesian_product_view::_Iterator::_Iterator): Remove
>         constraint on default constructor as per LWG 3849.
>         (cartesian_product_view::_Iterator::_M_prev): Adjust position
>         of _Nm > 0 test as per LWG 3820.
>         (cartesian_product_view::_Iterator::_M_advance): Perform bound
>         checking only on sized cartesian products.
>         * testsuite/std/ranges/cartesian_product/1.cc (test08): New test.
> ---
>  libstdc++-v3/include/std/ranges               | 23 +++++++++++--------
>  .../std/ranges/cartesian_product/1.cc         |  9 ++++++++
>  2 files changed, 22 insertions(+), 10 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> index 67566c6ebcf..14f38727198 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -8225,7 +8225,7 @@ namespace views::__adaptor
>                                     range_reference_t<__maybe_const_t<_Const, _Vs>>...>;
>      using difference_type = decltype(cartesian_product_view::_S_difference_type());
>
> -    _Iterator() requires forward_range<__maybe_const_t<_Const, _First>> = default;
> +    _Iterator() = default;
>
>      constexpr
>      _Iterator(_Iterator<!_Const> __i)
> @@ -8390,12 +8390,12 @@ namespace views::__adaptor
>      _M_prev()
>      {
>        auto& __it = std::get<_Nm>(_M_current);
> -      if (__it == ranges::begin(std::get<_Nm>(_M_parent->_M_bases)))
> -       {
> -         __it = __detail::__cartesian_common_arg_end(std::get<_Nm>(_M_parent->_M_bases));
> -         if constexpr (_Nm > 0)
> +      if constexpr (_Nm > 0)
> +       if (__it == ranges::begin(std::get<_Nm>(_M_parent->_M_bases)))
> +         {
> +           __it = __detail::__cartesian_common_arg_end(std::get<_Nm>(_M_parent->_M_bases));
>             _M_prev<_Nm - 1>();
> -       }
> +         }
>        --__it;
>      }
>
> @@ -8416,10 +8416,13 @@ namespace views::__adaptor
>           if constexpr (_Nm == 0)
>             {
>  #ifdef _GLIBCXX_ASSERTIONS
> -             auto __size = ranges::ssize(__r);
> -             auto __begin = ranges::begin(__r);
> -             auto __offset = __it - __begin;
> -             __glibcxx_assert(__offset + __x >= 0 && __offset + __x <= __size);
> +             if constexpr (sized_range<__maybe_const_t<_Const, _First>>)
> +               {
> +                 auto __size = ranges::ssize(__r);
> +                 auto __begin = ranges::begin(__r);
> +                 auto __offset = __it - __begin;
> +                 __glibcxx_assert(__offset + __x >= 0 && __offset + __x <= __size);
> +               }
>  #endif
>               __it += __x;
>             }
> diff --git a/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc b/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
> index f52c2b96d58..56ff3d152c6 100644
> --- a/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
> +++ b/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
> @@ -201,6 +201,14 @@ test07()
>    VERIFY( i == 5 );
>  }
>
> +void
> +test08()
> +{
> +  // LWG 3820
> +  auto r = std::views::cartesian_product(std::views::iota(0));
> +  r.begin() += 3; // hard error
> +}
> +
>  int
>  main()
>  {
> @@ -211,4 +219,5 @@ main()
>    test05();
>    static_assert(test06());
>    test07();
> +  test08();
>  }
> --
> 2.40.0.rc0.57.g454dfcbddf
>