On Wed, 16 Jul 2025, Tomasz Kaminski wrote:
>
>
> On Tue, Jul 15, 2025 at 9:51 PM Patrick Palka <ppa...@redhat.com> wrote:
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
>
> -- >8 --
>
> The offset-based partial specialization of _CachedPosition for
> random-access iterators is currently only selected if the offset type is
> smaller than the iterator type. Before r12-1018-g46ed811bcb4b86 this
> made sense since the main partial specialization only stored the
> iterator (incorrectly). After that bugfix, the main partial
> specialization now effectively stores a std::optional<iter>, so the
> size constraint is inaccurate,
>
> optional<Iterator> is still smaller than 2 * sizeof(Iterator), so the
> constraints
> could be updated to range::difference <= 2 * sizeof(Iterator) or <=
> sizeof(non_propagating_cache).
>
> However, having a range::difference_t being larger than iterator, should be
> unusual, as the iterator must be able to represent as many positions as the
> difference
> type. So except for cases like repeat, they should be similar size. I do not
> think
> it is worth optimizing for that.
>
> I think it is easier for users if the caching rules are simplified, and does
> not depend
> on size.
>
> and it must invalidate itself upon
> copy/move unlike the offset-based partial specialization. So I think
> we should just always prefer the offset-based _CachedPosition for a
> random-access iterator, even if the offset type happens to be larger
> than the iterator type.
>
>
> libstdc++-v3/ChangeLog:
>
> * include/std/ranges (__detail::_CachedPosition): Remove
> additional size constraint on the offset-based partial
> specialization.
> ---
>
> Given the above LGTM.
Thanks, Tomasz! Is this OK to push?
> libstdc++-v3/include/std/ranges | 2 --
> 1 file changed, 2 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/ranges
> b/libstdc++-v3/include/std/ranges
> index efe62969d657..2970b2cbe00b 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -1585,8 +1585,6 @@ namespace views::__adaptor
> };
>
> template<random_access_range _Range>
> - requires (sizeof(range_difference_t<_Range>)
> - <= sizeof(iterator_t<_Range>))
> struct _CachedPosition<_Range>
> {
> private:
> --
> 2.50.1.271.gd30e120486
>
>
>
