[PATCH] libstdc++: Add comparison operators between tuple<> and array<T, 0>

[Hewill Kang]

Wow, I just noticed that the current wording allows tuple<> to be compared
to any array<T, 0>, regardless of the type of T.
However, tuple<T> can only be compared to array<T, 1> if T itself is
comparable.
This seems to be a defect.

Osama Abdelkader <osama.abdelkader@gmail.com> 於 2025年10月17日 週五 下午11:27寫道：

> Fixes #119721
>
> This fixes a C++23 compliance issue where std::tuple<> cannot be compared
> with std::array<T, 0>. Both are empty tuple-like types and should be
> comparable according to the C++23 standard.
>
> The fix adds the missing comparison operators:
> - operator== and operator!= (return true and false respectively)
> - operator<, operator<=, operator>, operator>= (follow ordering rules for
> empty types)
> - operator<=> (returns strong_ordering::equal)
>
> This resolves the 'rejects-valid' bug where GCC was rejecting valid C++23
> code
> that Clang with libc++ already supports correctly.
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/tuple: Add comparison operators between tuple<> and
> array<T, 0>.
>
> Signed-off-by: Osama Abdelkader <osama.abdelkader@gmail.com>
> ---
>  libstdc++-v3/include/std/tuple | 88 ++++++++++++++++++++++++++++++++++
>  1 file changed, 88 insertions(+)
>
> diff --git a/libstdc++-v3/include/std/tuple
> b/libstdc++-v3/include/std/tuple
> index 0ca616f1b..d3621dd16 100644
> --- a/libstdc++-v3/include/std/tuple
> +++ b/libstdc++-v3/include/std/tuple
> @@ -1880,6 +1880,94 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>           using _Cat = typename
> __tuple_like_common_comparison_category<_UTuple>::type;
>           return std::__tuple_cmp<_Cat>(__t, __u,
> index_sequence_for<_Elements...>());
>         }
> +
> +  // Comparison operators between tuple<> and array<T, 0>
> +  // These are needed because both are empty tuple-like types in C++23
> +
> +  template<typename _Tp>
> +    [[nodiscard]]
> +    constexpr bool
> +    operator==(const tuple<>&, const array<_Tp, 0>&)
> +    { return true; }
> +
> +  template<typename _Tp>
> +    [[nodiscard]]
> +    constexpr bool
> +    operator==(const array<_Tp, 0>&, const tuple<>&)
> +    { return true; }
> +
> +  template<typename _Tp>
> +    [[nodiscard]]
> +    constexpr bool
> +    operator!=(const tuple<>&, const array<_Tp, 0>&)
> +    { return false; }
> +
> +  template<typename _Tp>
> +    [[nodiscard]]
> +    constexpr bool
> +    operator!=(const array<_Tp, 0>&, const tuple<>&)
> +    { return false; }
> +
> +  template<typename _Tp>
> +    [[nodiscard]]
> +    constexpr bool
> +    operator<(const tuple<>&, const array<_Tp, 0>&)
> +    { return false; }
> +
> +  template<typename _Tp>
> +    [[nodiscard]]
> +    constexpr bool
> +    operator<(const array<_Tp, 0>&, const tuple<>&)
> +    { return false; }
> +
> +  template<typename _Tp>
> +    [[nodiscard]]
> +    constexpr bool
> +    operator<=(const tuple<>&, const array<_Tp, 0>&)
> +    { return true; }
> +
> +  template<typename _Tp>
> +    [[nodiscard]]
> +    constexpr bool
> +    operator<=(const array<_Tp, 0>&, const tuple<>&)
> +    { return true; }
> +
> +  template<typename _Tp>
> +    [[nodiscard]]
> +    constexpr bool
> +    operator>(const tuple<>&, const array<_Tp, 0>&)
> +    { return false; }
> +
> +  template<typename _Tp>
> +    [[nodiscard]]
> +    constexpr bool
> +    operator>(const array<_Tp, 0>&, const tuple<>&)
> +    { return false; }
> +
> +  template<typename _Tp>
> +    [[nodiscard]]
> +    constexpr bool
> +    operator>=(const tuple<>&, const array<_Tp, 0>&)
> +    { return true; }
> +
> +  template<typename _Tp>
> +    [[nodiscard]]
> +    constexpr bool
> +    operator>=(const array<_Tp, 0>&, const tuple<>&)
> +    { return true; }
> +
> +  template<typename _Tp>
> +    [[nodiscard]]
> +    constexpr strong_ordering
> +    operator<=>(const tuple<>&, const array<_Tp, 0>&)
> +    { return strong_ordering::equal; }
> +
> +  template<typename _Tp>
> +    [[nodiscard]]
> +    constexpr strong_ordering
> +    operator<=>(const array<_Tp, 0>&, const tuple<>&)
> +    { return strong_ordering::equal; }
> +
>  #endif // C++23
>
>  #else // ! (concepts && consteval)
> --
> 2.43.0
>
>
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