[libcxx-commits] [libcxx] [libc++][ranges] implement `ranges::shift_left` (PR #83231)

[Louis Dionne via libcxx-commits]

================
@@ -24,30 +29,42 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 20
 
-template <class _ForwardIterator>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _ForwardIterator
-shift_left(_ForwardIterator __first,
-           _ForwardIterator __last,
-           typename iterator_traits<_ForwardIterator>::difference_type __n) {
+template <class _AlgPolicy, class _Iter, class _Sent>
+_LIBCPP_HIDE_FROM_ABI constexpr pair<_Iter, _Iter>
+__shift_left(_Iter __first, _Sent __last, typename _IterOps<_AlgPolicy>::template __difference_type<_Iter> __n) {
+  _LIBCPP_ASSERT_UNCATEGORIZED(__n >= 0, "__n must be greater than or equal to 0");
+
   if (__n == 0) {
-    return __last;
+    _Iter __end = _IterOps<_AlgPolicy>::next(__first, __last);
+    return {std::move(__first), std::move(__end)};
   }
 
-  _ForwardIterator __m = __first;
-  if constexpr (__has_random_access_iterator_category<_ForwardIterator>::value) {
-    if (__n >= __last - __first) {
-      return __first;
+  _Iter __m = __first;
+  if constexpr (sized_sentinel_for<_Sent, _Iter>) {
+    auto __size = _IterOps<_AlgPolicy>::distance(__first, __last);
+    if (__n >= __size) {
+      return {__first, std::move(__first)};
     }
-    __m += __n;
+    _IterOps<_AlgPolicy>::advance(__m, __n);
   } else {
     for (; __n > 0; --__n) {
       if (__m == __last) {
-        return __first;
+        return {__first, std::move(__first)};
       }
       ++__m;
     }
   }
-  return std::move(__m, __last, __first);
+
+  _Iter __result = std::__move<_AlgPolicy>(__m, __last, __first).second;
+  return {std::move(__first), std::move(__result)};
+}
+
+template <class _ForwardIterator>
+inline _LIBCPP_HIDE_FROM_ABI constexpr _ForwardIterator
----------------
ldionne wrote:

```suggestion
_LIBCPP_HIDE_FROM_ABI constexpr _ForwardIterator
```

https://github.com/llvm/llvm-project/pull/83231