Update libcxx/include/__vector/vector.h

Author: ldionne

libcxx/include/__vector/vector.h

@@ -1260,7 +1260,7 @@ vector<_Tp, _Allocator>::__insert_with_sentinel(const_iterator __position, _Inpu
     auto __guard = std::__make_exception_guard(
         _AllocatorDestroyRangeReverse<allocator_type, pointer>(__alloc_, __old_last, this->__end_));
     __v.__construct_at_end_with_sentinel(std::move(__first), std::move(__last));
-    __split_buffer<value_type, allocator_type&> __merged(__recommend(size() + __v.size()), __off, __alloc_);
+    __split_buffer<value_type, allocator_type&> __merged(__recommend(size() + __v.size()), __off, __alloc_); // has `__off` positions available at the front
     std::__uninitialized_allocator_relocate(
         __alloc_, std::__to_address(__old_last), std::__to_address(this->__end_), std::__to_address(__merged.__end_));
     __guard.__complete(); // Release the guard once objects in [__old_last_, __end_) have been successfully relocated.