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

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On Tue, 23 Jan 2024, 23:53 Patrick Palka, <ppalka@redhat.com> wrote:

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


OK



> 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
>
>