I use:
JetBrains ReSharper 2021.2.1 Build 212.0.20210826.105310 built on 2021-08-26
ReSharper 2021.2.20210826.112029
StyleCop.Resharper 2021.2.0, which is a fork of StyleCop Classic.

I asked to them to integrate the feature of C# 9, but they recommend me to report this issue here.
If I do null check of an object with "is not null", I got error warning from StyleCop. see attachment below.

Would you please integrate the features of C# 9 in your StyleCop classic.
Note: C# 9.0 is not very new since it is released on May 2020!
Thank you in advance.