Re: [PATCH] libstdc++: Add workaround to std::ranges::subrange [PR111948]

[Jonathan Wakely <jwakely@redhat.com>]

On Thu, 30 Nov 2023 at 15:52, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> I think I'll push this to work around the compiler bug. We can revert it
> later if the front end gets fixed.
>
> Tested x86_64-linux. Needed on trunk and gcc-13.

Pushed to trunk now.

>
> -- >8 --
>
> libstdc++-v3/ChangeLog:
>
>         PR libstdc++/111948
>         * include/bits/ranges_util.h (subrange): Add constructor to
>         _Size to aoid setting member in constructor.
>         * testsuite/std/ranges/subrange/111948.cc: New test.
> ---
>  libstdc++-v3/include/bits/ranges_util.h       | 21 ++++++++++++-------
>  .../testsuite/std/ranges/subrange/111948.cc   |  8 +++++++
>  2 files changed, 21 insertions(+), 8 deletions(-)
>  create mode 100644 libstdc++-v3/testsuite/std/ranges/subrange/111948.cc
>
> diff --git a/libstdc++-v3/include/bits/ranges_util.h b/libstdc++-v3/include/bits/ranges_util.h
> index ab6c69c57d0..185e46ec7a9 100644
> --- a/libstdc++-v3/include/bits/ranges_util.h
> +++ b/libstdc++-v3/include/bits/ranges_util.h
> @@ -267,13 +267,21 @@ namespace ranges
>        using __size_type
>         = __detail::__make_unsigned_like_t<iter_difference_t<_It>>;
>
> -      template<typename, bool = _S_store_size>
> +      template<typename _Tp, bool = _S_store_size>
>         struct _Size
> -       { };
> +       {
> +         [[__gnu__::__always_inline__]]
> +         constexpr _Size(_Tp = {}) { }
> +       };
>
>        template<typename _Tp>
>         struct _Size<_Tp, true>
> -       { _Tp _M_size; };
> +       {
> +         [[__gnu__::__always_inline__]]
> +         constexpr _Size(_Tp __s = {}) : _M_size(__s) { }
> +
> +         _Tp _M_size;
> +       };
>
>        [[no_unique_address]] _Size<__size_type> _M_size = {};
>
> @@ -294,11 +302,8 @@ namespace ranges
>        noexcept(is_nothrow_constructible_v<_It, decltype(__i)>
>                && is_nothrow_constructible_v<_Sent, _Sent&>)
>         requires (_Kind == subrange_kind::sized)
> -      : _M_begin(std::move(__i)), _M_end(__s)
> -      {
> -       if constexpr (_S_store_size)
> -         _M_size._M_size = __n;
> -      }
> +      : _M_begin(std::move(__i)), _M_end(__s), _M_size(__n)
> +      { }
>
>        template<__detail::__different_from<subrange> _Rng>
>         requires borrowed_range<_Rng>
> diff --git a/libstdc++-v3/testsuite/std/ranges/subrange/111948.cc b/libstdc++-v3/testsuite/std/ranges/subrange/111948.cc
> new file mode 100644
> index 00000000000..dcc64b56def
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/std/ranges/subrange/111948.cc
> @@ -0,0 +1,8 @@
> +// { dg-do compile { target c++20 } }
> +
> +#include <ranges>
> +
> +// Bug libstdc++/111948 - subrange modifies a const size object
> +
> +constexpr auto r = std::ranges::subrange(std::views::iota(0), 5);
> +static_assert(std::ranges::distance(r));
> --
> 2.43.0
>