namespace My.Lib;

[GenerateSerializer]
public class MyType<T>
{
    public MyType(OtherType original)
    {
      // ...
    }

    // tested attributes
    // [OrleansConstructor] // obsolete, but works
    // [GeneratedActivatorConstructor] // the migration path as specified in docs, but gives error 
    // [ActivatorUtilitiesConstructor] // workaround - solves the issue.

    [GeneratedActivatorConstructor] 
    public MyType(string a, string b, T? c)
    {
        // ...
    }

    [Id(0)]
    public string A { get; }
    [Id(1)]
    public string? B { get; }
    [Id(2)]
    public T? C { get; set; }
}