MemberNotNullWhen null tracking not working with generics

[kofifus]

Please refer me elsewhere if this is not the right place

using VS 16.8.0 Preview 2.1 with <TargetFramework>net5.0</TargetFramework><LangVersion>preview</LangVersion> <Nullable>enable</Nullable>

Consider first the non generic example that works correctly:

class GetResult {
    [MemberNotNullWhen(true, "Value")]
    public bool OK { get; set; }
    public Manager? Value { get; set; }
}

record Manager(int Age); 

class Archive {
  readonly Dictionary<string, Manager> Dict = new Dictionary<string, Manager>();

  public GetResult this[string key] 
    => Dict.TryGetValue(key, out var value)
          ? new GetResult {OK = true, Value = value}
          : new GetResult {OK = false, Value = null};
}

public class C {
    public void M() {
        Archive archive = new Archive();
        
        GetResult result = archive["John"];
        if (!result.OK) return;
        int age1 = result.Value.Age; // NO WARNING HERE - CORRECT
    }
}

Now almost the same code with generics:

class GetResult<T> {
    [MemberNotNullWhen(true, "Value")]
    public bool OK { get; set; }
    public T? Value { get; init; }
}

record Manager(int Age); 

class Archive {
  readonly Dictionary<string, Manager> Dict = new Dictionary<string, Manager>();

  public GetResult<Manager> this[string key] 
    => Dict.TryGetValue(key, out var value)
          ? new GetResult<Manager> {OK = true, Value = value}
          : new GetResult<Manager> {OK = false, Value = null};
}

public class C {
    public void M() {
        Archive archive = new Archive();
        
        var result = archive["John"];
        if (!result.OK) return;
        int age1 = result.Value.Age; // WARNING HERE - Dereference of a possibly null reference
    }
}

Seems like the compiler does not correctly null track the second generic version hence the warning Dereference of a possibly null reference

Any thoughts ?

[333fred]

Please open a bug on roslyn for this.

[jnm2]

https://github.com/dotnet/roslyn/issues/new?template=bug-report.md