Incorrect Generation of `default` Constraint

[JasonBock]

To reproduce:

public class BaseStuff
{
  public virtual T? GetService<T>(object[] args) where T : class => default!;
}

public static class Test
{
  public static void Go() => Rock.Create<BaseStuff>();
}

This will give the following error:

CS8822: Method 'CreateExpectationsOfBaseStuffExtensions.RockBaseStuff.GetService<T>(object[])' specifies a 'default' constraint for type parameter 'T', but corresponding type parameter 'T' of overridden or explicitly implemented method 'BaseStuff.GetService<T>(object[])' is constrained to a reference type or a value type.

This was found on System.Reactive.PlatformServices.CurrentPlatformEnlightenmentProvider.