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; };
> --
>
>
