For the record (i.e. so I don't have to remember/rediscover this), I tried
to backport this change to gcc-11 but it doesn't work on that branch
because of PR 68942 ("overly strict use of deleted function before
argument-dependent lookup (ADL)").

We can live with it being a gcc-12+ feature.


On Thu, 8 Sept 2022 at 19:30, Jonathan Wakely wrote:

> Tested powerpc64le-linux, pushed to trunk.
>
> -- >8 --
>
> The new proposed resolution for LWG 3629 says that std::error_code and
> std::error_condition should only use ADL to find their customization
> points. This means we need to use a poison pill to prevent lookup from
> finding overloads in the enclosing namespaces.
>
> We can also remove the forward declarations of std::make_error_code and
> std::make_error_condition, because they aren't needed now. ADL can find
> them anyway (when std is an associated namespace), and unqualified name
> lookup will not (and should not) find them.
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/system_error (__adl_only::make_error_code): Add
>         deleted function.
>         (__adl_only::make_error_condition): Likewise.
>         (error_code::error_code(ErrorCodeEnum)): Add using-declaration
>         for deleted function.
>         (error_condition::error_condition(ErrorConditionEnum)):
>         Likewise.
>         * testsuite/19_diagnostics/error_code/cons/lwg3629.cc: New test.
>         * testsuite/19_diagnostics/error_condition/cons/lwg3629.cc: New
> test.
> ---
>  libstdc++-v3/include/std/system_error         | 18 +++++--
>  .../19_diagnostics/error_code/cons/lwg3629.cc | 48 +++++++++++++++++++
>  .../error_condition/cons/lwg3629.cc           | 48 +++++++++++++++++++
>  3 files changed, 109 insertions(+), 5 deletions(-)
>  create mode 100644
> libstdc++-v3/testsuite/19_diagnostics/error_code/cons/lwg3629.cc
>  create mode 100644
> libstdc++-v3/testsuite/19_diagnostics/error_condition/cons/lwg3629.cc
>
> diff --git a/libstdc++-v3/include/std/system_error
> b/libstdc++-v3/include/std/system_error
> index 050439427cc..e12bb2f0e1e 100644
> --- a/libstdc++-v3/include/std/system_error
> +++ b/libstdc++-v3/include/std/system_error
> @@ -195,7 +195,11 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
>     * @{
>     */
>
> -  error_code make_error_code(errc) noexcept;
> +namespace __adl_only
> +{
> +  void make_error_code() = delete;
> +  void make_error_condition() = delete;
> +}
>
>    /** Class error_code
>     *
> @@ -231,7 +235,10 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
>      template<typename _ErrorCodeEnum,
>              typename = _Check<_ErrorCodeEnum>>
>        error_code(_ErrorCodeEnum __e) noexcept
> -      { *this = make_error_code(__e); }
> +      {
> +       using __adl_only::make_error_code;
> +       *this = make_error_code(__e);
> +      }
>
>      error_code(const error_code&) = default;
>      error_code& operator=(const error_code&) = default;
> @@ -330,8 +337,6 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
>      operator<<(basic_ostream<_CharT, _Traits>& __os, const error_code&
> __e)
>      { return (__os << __e.category().name() << ':' << __e.value()); }
>
> -  error_condition make_error_condition(errc) noexcept;
> -
>    /** Class error_condition
>     *
>     * This class represents error conditions that may be visible at an API
> @@ -363,7 +368,10 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
>      template<typename _ErrorConditionEnum,
>              typename = _Check<_ErrorConditionEnum>>
>        error_condition(_ErrorConditionEnum __e) noexcept
> -      { *this = make_error_condition(__e); }
> +      {
> +       using __adl_only::make_error_condition;
> +       *this = make_error_condition(__e);
> +      }
>
>      error_condition(const error_condition&) = default;
>      error_condition& operator=(const error_condition&) = default;
>
>