BCE0149 on external method that has generic argument constraint referencing declaring type's external constructed generic argument

using System.Collections.Generic;
public class D {
}
public class C<T> {
	public IEnumerable<U> Do<U> () where U : T
	{
		return null;
	}
}

class B:
    pass
C[of B]().Do[of B]() #B is internal, it works!

C[of D]().Do[of D]() #D is external -> BCE0149 :(

class B:
    pass
class D (B):
    pass

class C[of T]:
    def Do[of U(T)]() as U*:
        pass

C[of B]().Do[of B]()