Re: [PATCH] libstdc++: Workaround for LLVM-61763 in ranges

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On Sat, 14 Oct 2023, 00:33 Benjamin Brock, <brock@cs.berkeley.edu> wrote:

> This is my first time submitting a patch, so my apologies if I'm
> submitting incorrectly or missing something.
>

Thanks for contributing!

I don't think this patch counts as legally significant, but if you
contribute again in future you should be aware of
https://gcc.gnu.org/contribute.html#legal and either complete the copyright
assignment paperwork, or add a DCO sign-off to the commit message.


> Clang is unable to compile parts of libstdc++'s ranges implementation
> due to LLVM-61763, a Clang frontend compiler bug in handling the
> declarations of constrained friends.  The problem areas are zip_view,
> zip_transform_view, and adjacent_transform_view.
>
> A simple ranges program like the following fails to compile using
> Clang trunk and libstdc++.
>
>   std::vector v = {1, 2, 3, 4};
>   int sum = 0;
>   for (auto&& [i, j] : std::ranges::views::zip(v, v))
>     sum += i * j;
>
> In file included from <source>:1:
>
> /opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/14.0.0/../../../../include/c++/14.0.0/ranges:4655:14:
> error: type constraint differs in template redeclaration
>  4655 |     template<move_constructible _Fp, input_range... _Ws>
>       |              ^
> . . . . . .
>
> Godbolt: https://godbolt.org/z/Ynbs15aGh
>
> This patch adds a small workaround that avoids declaring constrained
> friends when compiling with Clang, instead making some members public.
> MSVC's standard library has implemented a similar workaround.
>
> Scanning through libstdc++, there do appear to be other workarounds
> for Clang, e.g. in complex and experimental/simd.  Hopefully this kind
> of workaround is acceptable---while the core issue is a Clang compiler
> bug, it may take a while to fix, and it would be very useful for
> libstdc++ ranges to work with Clang in the meantime.
>

Yes, this is ok because the hack is limited to __clang__.


> 2023-10-13  Benjamin Brock <brock@cs.berkeley.edu>
>
>     libstdc++-v3/ChangeLog:
>
>             * include/std/ranges: implement workaround for LLVM-61763 in
>               zip_view and adjacency_view
>

This should be a complete sentence, so capital letter and full stop.

I can get this pushed to trunk and gcc-13 next week, thanks again for the
patch.



> ---
>
> diff --git a/libstdc++-v3/include/std/ranges
> b/libstdc++-v3/include/std/ranges
> index 1d529a886be..7893e3a84c9 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -4632,6 +4632,9 @@ namespace views::__adaptor
>    class zip_view<_Vs...>::_Iterator
>      : public __detail::__zip_view_iter_cat<_Const, _Vs...>
>    {
> +#ifdef __clang__ // LLVM-61763 workaround
> +  public:
> +#endif
>
>  __detail::__tuple_or_pair_t<iterator_t<__detail::__maybe_const_t<_Const,
> _Vs>>...> _M_current;
>
>      constexpr explicit
> @@ -4652,11 +4655,13 @@ namespace views::__adaptor
>         return input_iterator_tag{};
>      }
>
> +#ifndef __clang__ // LLVM-61763 workaround
>      template<move_constructible _Fp, input_range... _Ws>
>        requires (view<_Ws> && ...) && (sizeof...(_Ws) > 0) &&
> is_object_v<_Fp>
>         && regular_invocable<_Fp&, range_reference_t<_Ws>...>
>         && std::__detail::__can_reference<invoke_result_t<_Fp&,
> range_reference_t<_Ws>...>>
>        friend class zip_transform_view;
> +#endif
>
>    public:
>      // iterator_category defined in __zip_view_iter_cat
> @@ -5327,6 +5332,9 @@ namespace views::__adaptor
>    template<bool _Const>
>    class adjacent_view<_Vp, _Nm>::_Iterator
>    {
> +#ifdef __clang__ // LLVM-61763 workaround
> +  public:
> +#endif
>      using _Base = __detail::__maybe_const_t<_Const, _Vp>;
>      array<iterator_t<_Base>, _Nm> _M_current = array<iterator_t<_Base>,
> _Nm>();
>
> @@ -5367,12 +5375,14 @@ namespace views::__adaptor
>
>      friend class adjacent_view;
>
> +#ifndef __clang__ // LLVM-61763 workaround
>      template<forward_range _Wp, move_constructible _Fp, size_t _Mm>
>        requires view<_Wp> && (_Mm > 0) && is_object_v<_Fp>
>          && regular_invocable<__detail::__unarize<_Fp&, _Mm>,
> range_reference_t<_Wp>>
>          &&
> std::__detail::__can_reference<invoke_result_t<__detail::__unarize<_Fp&,
> _Mm>,
>
> range_reference_t<_Wp>>>
>        friend class adjacent_transform_view;
> +#endif
>
>    public:
>      using iterator_category = input_iterator_tag;
>