Generic method returning generic interface in Delphi 2010

Question

Given the code below, wich is a very trimmed down version of the actual code, I get the following error:

[DCC Error] Unit3.pas(31): E2010 Incompatible types: \'IXL

Answer 1

With three minor modification (IInterface, FindAll with "S: class" [Thanks Mason] and the typecasts in FindAll) I got it compiling.

Full code:

unit Unit16;

interface

uses
  Generics.Collections;

type
  IXList<T> = interface
  end;

  TXList<T: class> = class(TList<T>, IInterface, IXList<T>)
  protected
    FRefCount: Integer;
    function QueryInterface(const IID: TGUID; out Obj): HResult; stdcall;
    function _AddRef: Integer; stdcall;
    function _Release: Integer; stdcall;
  public
    function Find: IXList<T>;
    function FindAll<S: class>: IXList<S>;
  end;

implementation
uses Windows;

function TXList<T>.Find: IXList<T>;
begin
  Result := TXList<T>.Create;
end;

function TXList<T>.FindAll<S>: IXList<S>;
begin
  Result := IXList<S>(IUnknown(TXList<S>.Create));
end;

function TXList<T>.QueryInterface(const IID: TGUID; out Obj): HResult;
begin
  Result := E_NoInterface;
end;

function TXList<T>._AddRef: Integer;
begin
  InterlockedIncrement(FRefCount);
end;

function TXList<T>._Release: Integer;
begin
  InterlockedDecrement(FRefCount);
  if FRefCount = 0 then Self.Destroy;
end;

end.

Answer 2

That definitely looks like a compiler error. They're saying how they've focused a lot of effort into improving Generics issues for the next version, Delphi XE. When it gets released, which should be within the next couple weeks, download the preview and see if that will compile now. If not, try filing a bug report with QC.

Also, FindAll<S> should probably be declared as function FindAll<S: class>: IXList<S>;. That doesn't fix the error, but a working compiler would probably give you an error on that.