MaybeNullAttribute not working

[JamesNK]

I'm converting Newtonsoft.Json to use nullable types and I can't get MaybeNullAttribute to work. My remaining warnings are related to generic types.

Note that I have many old targets so I'm using internal nullable attributes (located in NullableAttributes.cs). NotNullAttribute and NotNullWhenAttribute work fine, but I can't get MaybeNullAttribute to work. I've tried it in System.Runtime.CompilerServices and System.Diagnostics.CodeAnalysis namespaces with no result.

Version Used: VS2019 + Microsoft.Net.Compilers.Toolset=3.2.0-beta3-final

Steps to Reproduce:

1. git clone https://github.com/JamesNK/Newtonsoft.Json.git
2. git checkout jamesnk/nullable-types
3. Go to JsonConvert.DeserializeObject method (JsonConvert.cs, line 787)

Expected Behavior:

There should not be a nullable warning because [return: MaybeNull] is on the return type.

Actual Behavior:

There is a warning casting object? to T.

[RikkiGibson]

I think this example should not warn:

#nullable enable
using System.Diagnostics.CodeAnalysis;

namespace System.Diagnostics.CodeAnalysis
{
    class MaybeNullAttribute : System.Attribute { }
}

public class C {
    [return: MaybeNull]
    public T M<T>(object? o) {
        return (T)o; // gives warning CS8601, but probably shouldn't
    }
}

/cc @jcouv

[JamesNK]

This also shouldn't warn:

public class C {
    [return: MaybeNull]
    public T M<T>() {
        return default;
    }
}

[jcouv]

Thanks for reporting this.
This is currently by-design. The attributes ([AllowNull] and such) currently only affect callers. They do not affect implementations or OHI yet. This is tracked by #36039

[JamesNK]

What should I do in this situation? Suppress CS8601?

[jcouv]

Use the suppression operator. For example: return default!;.