[PATCH 1/2] libstdc++/pair: Define _S_const_assignable helper for C++20

[Patrick Palka]

This is consistent with std::tuple's __const_assignable member function,
and will be reused when implementing the new pair::operator= overloads
from P2165R4.

libstdc++-v3/ChangeLog:

        * include/bits/stl_pair.h (pair::_S_const_assignable): Define,
        factored out from ...
        (pair::operator=): ... the constraints of the const overloads.
---
 libstdc++-v3/include/bits/stl_pair.h | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/libstdc++-v3/include/bits/stl_pair.h 
b/libstdc++-v3/include/bits/stl_pair.h
index acd0c7b58f9..b81b479ad43 100644
--- a/libstdc++-v3/include/bits/stl_pair.h
+++ b/libstdc++-v3/include/bits/stl_pair.h
@@ -404,6 +404,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
          return false;
        }
 
+      template<typename _U1, typename _U2>
+       static constexpr bool
+       _S_const_assignable()
+       {
+         if constexpr (is_assignable_v<const _T1&, _U1>)
+           return is_assignable_v<const _T2&, _U2>;
+         return false;
+       }
+
       template<typename _U1, typename _U2>
        static constexpr bool
        _S_nothrow_assignable()
@@ -468,8 +477,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       /// Copy assignment operator (const)
       constexpr const pair&
       operator=(const pair& __p) const
-      requires is_copy_assignable_v<const first_type>
-       && is_copy_assignable_v<const second_type>
+      requires (_S_const_assignable<const first_type&, const second_type&>())
       {
        first = __p.first;
        second = __p.second;
@@ -479,8 +487,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       /// Move assignment operator (const)
       constexpr const pair&
       operator=(pair&& __p) const
-      requires is_assignable_v<const first_type&, first_type>
-       && is_assignable_v<const second_type&, second_type>
+      requires (_S_const_assignable<first_type, second_type>())
       {
        first = std::forward<first_type>(__p.first);
        second = std::forward<second_type>(__p.second);
@@ -491,8 +498,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       template<typename _U1, typename _U2>
        constexpr const pair&
        operator=(const pair<_U1, _U2>& __p) const
-       requires is_assignable_v<const first_type&, const _U1&>
-         && is_assignable_v<const second_type&, const _U2&>
+       requires (_S_const_assignable<const _U1&, const _U2&>())
        {
          first = __p.first;
          second = __p.second;
@@ -503,8 +509,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       template<typename _U1, typename _U2>
        constexpr const pair&
        operator=(pair<_U1, _U2>&& __p) const
-       requires is_assignable_v<const first_type&, _U1>
-         && is_assignable_v<const second_type&, _U2>
+       requires (_S_const_assignable<_U1, _U2>())
        {
          first = std::forward<_U1>(__p.first);
          second = std::forward<_U2>(__p.second);
-- 
2.43.0.386.ge02ecfcc53