[PATCH v2] libstdc++: Remove dead code in range_formatter::format [PR109162]

[Jonathan Wakely]

OK for trunk with two spelling fixes...

On Thu, 17 Apr 2025, 07:31 Tomasz Kamiński, <tkaminsk@redhat.com> wrote:

> Because the _M_format(__rg, __fc) were placed outside of if constexpr,
> these method and it's childs where



"its children"

instantiated, even if _M_format<const _Range>
> could be used.
>
> To simplify the if constexpr chain, we introduce a
> __simply_formattable_range
> (name based on simple-view) exposition only concept, that checks if range
> is
> const and mutable formattable and uses same formatter specialization for
> references in each case.
>
>         PR libstdc++/109162
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/format (__format::__simply_formattable_range):
> Define.
>         (range_formatter::format): Do not instantiate _M_format for mutable
>          _Rg if const _Rg can be used.
>
> Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
> ---
> I was really unhappy with repeated ifs, so introduced an helper concept,
> that I think is usable in general.
>
>  libstdc++-v3/include/std/format | 19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/format
> b/libstdc++-v3/include/std/format
> index 27253f50ea8..1780951b61d 100644
> --- a/libstdc++-v3/include/std/format
> +++ b/libstdc++-v3/include/std/format
> @@ -5252,6 +5252,14 @@ namespace __format
>        = ranges::input_range<const _Rg>
>           && formattable<ranges::range_reference_t<const _Rg>, _CharT>;
>
> +  // _Rg& and const _Rg& are both formttable and use same formatter
>

"formattable"

+  // specialization for their references.
> +  template<typename _Rg, typename _CharT>
> +    concept __simply_formattable_range
> +      = __const_formattable_range<_Rg, _CharT>
> +         && same_as<remove_cvref_t<ranges::range_reference_t<_Rg>>,
> +                    remove_cvref_t<ranges::range_reference_t<const _Rg>>>;
> +
>    template<typename _Rg, typename _CharT>
>      using __maybe_const_range
>        = __conditional_t<__const_formattable_range<_Rg, _CharT>, const
> _Rg, _Rg>;
> @@ -5635,13 +5643,10 @@ namespace __format
>         format(_Rg&& __rg, basic_format_context<_Out, _CharT>& __fc) const
>         {
>           using _Range = remove_reference_t<_Rg>;
> -         if constexpr (__format::__const_formattable_range<_Range,
> _CharT>)
> -         {
> -           using _CRef = ranges::range_reference_t<const _Range>;
> -           if constexpr (same_as<remove_cvref_t<_CRef>, _Tp>)
> -             return _M_format<const _Range>(__rg, __fc);
> -         }
> -         return _M_format(__rg, __fc);
> +         if constexpr (__format::__simply_formattable_range<_Range,
> _CharT>)
> +           return _M_format<const _Range>(__rg, __fc);
> +         else
> +           return _M_format(__rg, __fc);
>         }
>
>      private:
> --
> 2.49.0
>
>
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