AllowNullAttribute not working

[JamesNK]

Similar to #36986

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I'm converting Newtonsoft.Json to use nullable types and I can't get AllowNullAttribute 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 AllowNullAttribute 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 JsonConverter<T>.WriteJson method (JsonConverter.cs, line 97)

Expected Behavior:

There should not be a nullable warning because [AllowNull] is on the parameter type.

Actual Behavior:

There is a warning casting object? to T.

[jcouv]

The warning seems to be on converting from object? to T (which I assume is unconstrained).
Here's a self-contained repro which hopefully represents your situation: sharplab.
This is currently by-design and the only mitigation is to suppress with !. If T were substituted with non-nullable type string, then the warning would be there. So it should be there for T before substitution.

[jcouv]

Updated notes above. This is by-design (assigning maybe-null value to a T which may be non-nullable, such as string).