CS8604 False-negative in .Net Standard project vs .Net Core

Describe the bug

In a .Net Standard project, trying to use a Stream? instead of a Stream raises a CS8604 warning. But not when using a Stream? to initialize a StreamReader. In a .Net Standard project, both situations raise a CS8604 warning.

Steps To Reproduce

Here is the short class that I am using, with <Nullable>enable</Nullable> in .NetStandard 2.1 project and .NetCoreApp 3.1 project.

public class Test
{
    public async Task<Stream?> GetFile(string file)
    {
        Stream? stream = null;

        if (File.Exists(file))
        {
            stream = File.OpenRead(file);
        }

        return await Task.FromResult(stream);
    }

    public async Task TestStreamReader(string file)
    {
        using(Stream stream = await GetFile(file))
        {
            // Do stuff...
        }

        using(StreamReader reader = new StreamReader(await GetFile(file)))
        {
            // Do stuff...
        }
    }
}

Expected behavior

CS8604 for both using Stream and StreamReader in .Net Standard and .Net Core

Actual behavior

CS8604 for Stream in .Net Standard and for Stream and StreamReader in .Net Core

Analyzer

Package: Microsoft.CodeAnalysis.FxCopAnalyzers

Version: v3.3.0 (Latest)

Diagnostic ID: CA1716

Additional context

I understand this might be “intentional” as it should only work properly in .Net Core App if I understand this correctly but I feel like the fact that this is “half” working in .Net Standard can make it confusing.

[svick]

Am I correct to assume that with net5.0 on its way those annotations will not find their way to netstandard2.1 ?

Yes, but there is also a project that lets you copy annotations from one target framework to another.

[RikkiGibson]

Correct, there are no plans that I am aware of to introduce nullability annotations to netstandard2.1.