[PATCH] libstdc++: Fix condition for memoizing reverse_view::begin() [PR100621]

[Jonathan Wakely]

On 17/05/21 12:17 -0400, Patrick Palka via Libstdc++ wrote:
>A range being a random access range is not a sufficient condition for
>ranges::next(iter, sent) to have constant time complexity; the range
>must also have a sized sentinel.  This adjusts the memoization condition
>for reverse_view accordingly.
>
>Tested on x86_64-pc-linxu-gnu, does this look OK for trunk?  Doesn't
>seem to be worth backporting.

OK for trunk. I agree the backports probably aren't needed, but if
it causes anybody problems we can do it later.


>libstdc++-v3/ChangeLog:
>
>	* include/std/ranges (reverse_view::_S_needs_cached_begin):
>	Set to false if the underlying non-common random-access range
>	doesn't have a sized sentinel.
>---
> libstdc++-v3/include/std/ranges | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
>diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
>index bf52074ca05..e93469ca3b4 100644
>--- a/libstdc++-v3/include/std/ranges
>+++ b/libstdc++-v3/include/std/ranges
>@@ -3340,7 +3340,9 @@ namespace views::__adaptor
>     {
>     private:
>       static constexpr bool _S_needs_cached_begin
>-	= !common_range<_Vp> && !random_access_range<_Vp>;
>+	= !common_range<_Vp> && !(random_access_range<_Vp>
>+				  && sized_sentinel_for<sentinel_t<_Vp>,
>+							iterator_t<_Vp>>);
>
>       [[no_unique_address]]
> 	__detail::__maybe_present_t<_S_needs_cached_begin,
>-- 
>2.31.1.621.g97eea85a0a
>