unit U1;
interface
type

  TClass_1 = class
    public 
      f : TClass_2
  end;

implementation
end.

unit U2;
interface
type

  TClass_2 = class
    public 
      f : TClass_1
  end;

implementation
end.

type
{ Forward declarations }
  TFields = class;

{ Forward declarations }

  TFields = class;
  TField = class;

{ TFieldsEnumerator }

  TFieldsEnumerator = class
  private
    FIndex: Integer;
    FFields: TFields;
  public
    constructor Create(AFields: TFields);
    function GetCurrent: TField;
    function MoveNext: Boolean;
    property Current: TField read GetCurrent;
  end;

{ TFields }

  TFields = class(TObject)
  private

 TAbstractStrangeField = class(TComponent)
  end;

  TAbstractStrangeCollection<T: TAbstractStrangeField> = class
  private
    function GetField(Index: Integer): T;
    procedure SetField(Index: Integer; const Value: T);
    function GetCount: Integer;
  public
    property Count: Integer read GetCount;
    property Fields[Index: Integer]: T read GetField write SetField; default;
  end;

  TAbstractFieldsEnumerator<T: TAbstractStrangeField> = class
  private
    FIndex: Integer;
    FFields: TAbstractStrangeCollection<T>;
  public
    constructor Create(AFields: TAbstractStrangeCollection<T>);
    function GetCurrent: T;
    function MoveNext: Boolean;
    property Current: T read GetCurrent;
  end;

  TField = class(TAbstractStrangeField)
  end;

  TConcrateEnumerator = class(TAbstractFieldsEnumerator<TField>)
  end;

  TFields = class(TAbstractStrangeCollection<TField>)
  public
    function GetEnumerator: TConcrateEnumerator;
  end;

procedure TForm1.FormCreate(Sender: TObject);
var
  A: TFields;
  B: TField;
begin
  for B in A do
    ;
end;