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

A range being a random access range isn't a sufficient condition for
ranges::next(iter, sent) to have constant time complexity -- it must
also have a sized sentinel.  This adjusts the memoization condition for
reverse_view accordingly.

libstdc++-v3/ChangeLog:

	PR libstdc++/100621
	* include/std/ranges (reverse_view::_S_needs_cached_begin):
	Set to true if the underlying non-common random-access range
	doesn't have a sized sentinel.

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 36bccd6..7079425 100644 (file)
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -3228,7 +3228,9 @@ 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,