Remove redundant partial specialization in _Nth_type

[Feng Jisen]

This patch remove a redundant partial specialization in class _Nth_type.

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; };
--


From 6443b75b1097ed3d1a691535a4c5e9e9a96a935a Mon Sep 17 00:00:00 2001
From: J1senn <j1s...@outlook.com>
Date: Sat, 28 Oct 2023 20:49:42 +0800
Subject: [PATCH] remove redundant partial template in _Nth_type

---
 libstdc++-v3/include/bits/utility.h | 4 ----
 1 file changed, 4 deletions(-)

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; };
-- 
2.42.0