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

[Patrick Palka via Gcc-patches]

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.

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