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
