Improve __assign_with_sentinel in std::vector

Author: winner245

libcxx/include/__vector/vector.h

@@ -1031,9 +1031,14 @@ template <class _Tp, class _Allocator>
 template <class _Iterator, class _Sentinel>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void
 vector<_Tp, _Allocator>::__assign_with_sentinel(_Iterator __first, _Sentinel __last) {
-  clear();
-  for (; __first != __last; ++__first)
-    emplace_back(*__first);
+  pointer __cur = __begin_;
+  for (; __first != __last && __cur != __end_; ++__cur, ++__first)
+    *__cur = *__first;
+  if (__cur != __end_)
+    __destruct_at_end(__cur);
+  else
+    for (; __first != __last; ++__first)
+      emplace_back(*__first);
 }
 
 template <class _Tp, class _Allocator>