A simple example:

enum ControlType { Foo }

class A
{
    public ControlType ControlType = ControlType.Foo;
}

class B
{
    public ControlType? ControlType = ControlType.Foo; // <-- error CS0236: A field initializer cannot reference the non-static field, method, or property 'B.ControlType'
}

Why does A work, but B fails to compile?

Is it a bug? If not, where this different behavior is described/specified?