CS0592 for NotNullWhen on extension property

[UniqeId6542]

Version Used: VS 17.14.21 .NET 9.0.11 Preview language features (Also VS 18, .NET 10)

I have a custom error type where I use null to indicate success.

public static bool IsSuccess([NotNullWhen(false)] this MyErrorClass? info)
{
    return info is null;
}

With the new C# 14 features, I wanted to change this to a property.

extension(MyErrorClass? info)
{
    public bool IsSuccess
    {
        [NotNullWhen(false)] // CS0592 error here
        get => info is null;
    }
}

With the new extension properties, it is now possible to write a property that runs on a null object. So, null analysis should be updated to support this.

Expected Behavior:
Some way to tell the null analysis that the extended class is not null given some property return value.

Actual Behavior:
It is not possible to change from an extension method to an extension property while preserving the null analysis.

[RikkiGibson]

Place the attribute on the parameter instead, e.g. extension([NotNullWhen(false)] MyErrorClass? info). .NET Lab.

You can declare multiple extension blocks for the same type. This allows you to customize the this parameter for different extension members as appropriate.

[UniqeId6542]

While that does make it work, that adds an intellisense warning (not a compiler warning) - CS8602 Dereference of a possibly null reference.
That warning is mentioned in: https://developercommunity.visualstudio.com/t/extension-blocks-do-not-propogate-nullab/10952462?sort=newest

Is that considered a Visual Studio issue, or Rosyln issue?

[RikkiGibson]

That would be a Roslyn issue.

I wasn't able to repro this on https://lab.razor.fyi. I'm unsure what Intellisense warning but not a compiler warning means. Does that mean that you see the warning in the editor, but not in the command line build?

I would recommend trying again in VS 18.0 Insiders, as any extension blocks support in 17.14 would be quite out of date.