type
  TFoo = class

type
  TxRegion = class(TFoo)

type
  TFooCollection<T: TFoo> = class(TObjectDictionary<Integer, T>);

type
  TxRegionCollection<T: TxRegion> = class(TFooCollection<T>);

...
class function TfrmBaseDict.GetDictionary(AList: TFooCollection<TFoo>): Boolean;

Вызываю процедуру:
...
var
ARegionCollection: TxRegionsList<TxRegion>;

begin
 ARegionCollection:= TxdsRegionsList.GetAll;
-- Ошибка приведения типов
if TfrmReasonDict.GetDictionary(ARegionCollection) then


end;

type
  TxdsRegionsList = class(TObject)
  private
    { private declarations }
  protected
    { protected declarations }
  public
    { public declarations }
    class function GetAll: TxRegionsList<TxRegion>;

  if TfrmReasonDict.GetDictionary(TFooCollection<TFoo>(ARegionsList))

var
ARegionCollection: TxRegionCollection<TxRegion>;

 function FillDataset < T: TFoo > (AList:TFooCollection<T>): TdxMemData; virtual;

E2533 Virtual, dynamic and message methods cannot have type parameters (Delphi)

type

TfrmBaseDictionaryList = class(TForm)
MainDataList: TDictionaryList<TDictionary>;
class function GetDictionary < T: TDictionary > (AList: TDictionaryList<T>): Boolean;
end;
...

class function TfrmBaseDictionaryList.GetDictionary<T>(AList: TDictionaryList<T>): Boolean;
var
  AfrmBaseDictionaryList: TfrmBaseDictionaryList;
begin
  Result := False;
  Application.CreateForm(TfrmBaseDictionaryList, AfrmBaseDictionaryList);
  try
    AfrmBaseDictionaryList.MainDataList := AList;
...

 AfrmBaseDictionaryList.MainDataList := TDictionaryList<TDictionary>(AList);