Nullability information is wrong for lambdas in certain situations

[davidfowl]

Version Used: Version 17.0.0 Preview 3.0 [31528.426.main]

Steps to Reproduce:

using System.Reflection;

var dump = static (string s, Delegate d) =>
{
    var mi = d.Method;

    var context = new NullabilityInfoContext();
    var info = context.Create(mi.GetParameters()[0]);
    return new
    {
        ReadState = info.ReadState.ToString(),
        WriteState = info.WriteState.ToString(),
        Type = info.Type.ToString(),
        ElementType = info.ElementType?.ToString()
    };
};

Console.WriteLine(dump("/", (string? name) => $"Inline lambda {name}"));
Console.WriteLine(dump("/o", (string? name) => { }));

Expected Behavior:

{ ReadState = Nullable, WriteState = Nullable, Type = System.String, ElementType =  }
{ ReadState = Nullable, WriteState = Nullable, Type = System.String, ElementType =  }

Actual Behavior:

{ ReadState = Unknown, WriteState = Unknown, Type = System.String, ElementType =  }
{ ReadState = Unknown, WriteState = Unknown, Type = System.String, ElementType =  }

cc @cston

[halter73]

Adding a method to the sample that isn't void-returning and parameterless yields the expected behavior:

// This uses the same dump method from the original issue

Console.WriteLine(dump("/", (string? name) => $"Inline lambda {name}"));
Console.WriteLine(dump("/o", (string? name) => { }));

class RandomClass
{
    object RandomMethod() => throw new NotImplementedException();
}

{ ReadState = Nullable, WriteState = Nullable, Type = System.String, ElementType =  }
{ ReadState = Nullable, WriteState = Nullable, Type = System.String, ElementType =  }

Whereas adding a void-returning and parameterless method doesn't help:

class RandomClass
{
    void RandomMethod() => throw new NotImplementedException();
}

{ ReadState = Unknown, WriteState = Unknown, Type = System.String, ElementType =  }
{ ReadState = Unknown, WriteState = Unknown, Type = System.String, ElementType =  }

And adding a local function makes it half-work:

object RandomMethod() => throw new NotImplementedException();

{ ReadState = Nullable, WriteState = Nullable, Type = System.String, ElementType =  }
{ ReadState = Unknown, WriteState = Unknown, Type = System.String, ElementType =  }

You can see the start of this investigation here: https://github.com/dotnet/aspnetcore/pull/35233/files#r686358964

[cston]

As @halter73 discovered, the compiler is dropping [Nullable] attributes from the synthesized methods emitted for lambdas (and local functions?) if there are no explicit type declarations that require annotations.

The synthesized methods in the following are not annotated either:

class Program
{
    static void Main(string[] args)
    {
#nullable enable
        object? f() => null;
        var a = (string? s) => { };
    }
}