Remove redundant partial specialization in _Nth_type

[Jonathan Wakely]

On Sat, 28 Oct 2023 at 14:15, Feng Jisen wrote:
>
> This patch remove a redundant partial specialization in class _Nth_type.

Thanks for the patch, I've pushed it to trunk now.


>
> For the original metafunction _Nth_type code,
>   # 0
>   template<typename _Tp0, typename... _Rest>
>     struct _Nth_type<0, _Tp0, _Rest...>
>     { using type = _Tp0; };
>  # 1
>   template<typename _Tp0, typename _Tp1, typename... _Rest>
>     struct _Nth_type<0, _Tp0, _Tp1, _Rest...>
>     { using type = _Tp0; };
>   # 2
>   template<typename _Tp0, typename _Tp1, typename _Tp2, typename... _Rest>
>     struct _Nth_type<0, _Tp0, _Tp1, _Tp2, _Rest...>
>     { using type = _Tp0; };
>   # 3
>   template<size_t _Np, typename _Tp0, typename _Tp1, typename _Tp2,
>          typename... _Rest>
> #if __cpp_concepts
>     requires (_Np >= 3)
> #endif
>     struct _Nth_type<_Np, _Tp0, _Tp1, _Tp2, _Rest...>
>     : _Nth_type<_Np - 3, _Rest...>
>     { };
>
> we need partial specialization # 2 to deal with template argument <0, Tp0, Tp1, Tp2, ...>.
> Because without concepts, both # 0 and # 3 is legal and there is no partial order relationship between them.
> However, # 1 is redundant. For template argument <0, Tp0, Tp1>, #0 is instantiated and that's enough.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/utility.h:(_Nth_type) Remove redundant partial specialization.
>
>
> diff --git a/libstdc++-v3/include/bits/utility.h b/libstdc++-v3/include/bits/utility.h
> index bed94525642..8766dfbc15f 100644
> --- a/libstdc++-v3/include/bits/utility.h
> +++ b/libstdc++-v3/include/bits/utility.h
> @@ -258,10 +258,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      { };
>
>  #if ! __cpp_concepts // Need additional specializations to avoid ambiguities.
> -  template<typename _Tp0, typename _Tp1, typename... _Rest>
> -    struct _Nth_type<0, _Tp0, _Tp1, _Rest...>
> -    { using type = _Tp0; };
> -
>    template<typename _Tp0, typename _Tp1, typename _Tp2, typename... _Rest>
>      struct _Nth_type<0, _Tp0, _Tp1, _Tp2, _Rest...>
>      { using type = _Tp0; };
> --
>
>