On 21/02/2020 20:29, Patrick Palka wrote:
diff --git a/libstdc++-v3/include/bits/ranges_algo.h
b/libstdc++-v3/include/bits/ranges_algo.h
index 7de1072abf0..c36afc6e19b 100644
--- a/libstdc++-v3/include/bits/ranges_algo.h
+++ b/libstdc++-v3/include/bits/ranges_algo.h
@@ -3683,6 +3683,54 @@ namespace ranges
inline constexpr __prev_permutation_fn prev_permutation{};
} // namespace ranges
+
+ template<class ForwardIterator>
+ constexpr ForwardIterator
+ shift_left(ForwardIterator __first, ForwardIterator __last,
+ typename iterator_traits<ForwardIterator>::difference_type __n)
+ {
+ __glibcxx_assert(__n >= 0);
+ if (__n == 0)
+ return __last;
+
+ auto __mid = ranges::next(__first, __n, __last);
+ if (__mid == __last)
+ return __first;
+ return std::move(std::move(__mid), std::move(__last),
std::move(__first));
+ }
+
+ template<class ForwardIterator>
+ constexpr ForwardIterator
+ shift_right(ForwardIterator __first, ForwardIterator __last,
+ typename iterator_traits<ForwardIterator>::difference_type __n)
+ {
+ __glibcxx_assert(__n >= 0);
+ if (__n == 0)
+ return __first;
+
+ using _Cat = iterator_traits<ForwardIterator>::iterator_category;
^ FYI, the above line causes recent Clang 10 trunk with -std=c++20 to
fail due to a "missing" typedef
+ if constexpr (derived_from<_Cat, bidirectional_iterator_tag>)
+ {
+ auto __mid = ranges::next(__last, -__n, __first);
+ if (__mid == __first)
+ return __last;
+ return std::move_backward(std::move(__first), std::move(__mid),
+ std::move(__last));
+ }
+ else
+ {
+ auto __result = ranges::next(__first, __n, __last);
+ if (__result == __last)
+ return __last;
+ auto __dest = __result;
+ do
+ __dest = ranges::swap_ranges(__first, __result,
+ std::move(__dest), __last).in2;
+ while (__dest != __last);
+ return __result;
+ }
+ }
+
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
#endif // concepts
