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libstdc++/pair: Factor out const-assignability helper for C++20

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This is consistent with std::tuple's __const_assignable helper, and will
be useful for 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.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
This commit is contained in:
Patrick Palka 2024-02-01 14:56:02 -05:00
parent b14209715e
commit 87d162383e
1 changed files with 13 additions and 8 deletions
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21
libstdc++-v3/include/bits/stl_pair.h
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@ -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);