[PATCH WIP 2/2] libstdc++: Optimize ranges::distance for segmented iterators [PR123211]

[Patrick Palka]

For segmented iterators, ranges::distance is equivalent to the sum of
ranges::distance of each of its segments.

        PR libstdc++/123211

libstdc++-v3/ChangeLog:

        * include/bits/ranges_base.h (__distance_fn::operator()): For
        the non-sized-sentinel overload,
---
 libstdc++-v3/include/bits/ranges_base.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/libstdc++-v3/include/bits/ranges_base.h 
b/libstdc++-v3/include/bits/ranges_base.h
index ce1c40fc17f6..7f169feaa6b1 100644
--- a/libstdc++-v3/include/bits/ranges_base.h
+++ b/libstdc++-v3/include/bits/ranges_base.h
@@ -994,6 +994,16 @@ namespace ranges
       constexpr iter_difference_t<_It>
       operator()[[nodiscard]](_It __first, _Sent __last) const
       {
+       if constexpr (__segmented_iterator<_It> && same_as<_It, _Sent>)
+         {
+           iter_difference_t<_It> __n = 0;
+           std::__for_each_segment(__first, __last, [this, &__n](auto __first, 
auto __last) {
+             __n += iter_difference_t<_It>(this->operator()(__first, __last));
+             return __last;
+           });
+           return __n;
+         }
+
        iter_difference_t<_It> __n = 0;
        while (__first != __last)
          {
-- 
2.55.0.141.g55526a1826