Extension class compiles as long as there is no metaclass defined on TFoo.
If there is a metaclass TFooClass defined, E44 appears in extension class.
TFooClass = class of TFoo;
TFoo = public class(object)
function Bar: String; begin result := ‘Bar’; end;
TFooHelper = public extension class(TFoo)
function TwoBars: String;
result := self.Bar + self.Bar; // -> (E44) No member “Bar” on type “class of TFooHelper”