Generics still have a lot of problems. I know they've been in the works for years but they still fail easily. Consider the Delphi mode below, I get "Error: Generics without specialization cannot be used as a type for a variable"

I could easily write up a whole bunch of test cases were generic still fail, both in Fpc and Delphi modes. If someone is considering working on the compiler to fix the problems with generics I can happily send them a bunch of test cases where the compiler currently fails.

{$mode delphi}

{ TEnumerator<T> }

type
  TEnumerator<T> = class abstract
  protected
    function DoGetCurrent: T; virtual; abstract;
    function DoMoveNext: Boolean; virtual; abstract;
  public
    property Current: T read DoGetCurrent;
    function MoveNext: Boolean;
  end;

{ TEnumerable<T> }

  TEnumerable<T> = class abstract
  protected
    function DoGetEnumerator: TEnumerator<T>; virtual; abstract; // Error:
Generics without specialization cannot be used as a type for a variable
  public
    function GetEnumerator: TEnumerator<T>;
  end;

It will compile correctly with the trunk version.

Generics support has been greatly improved in the trunk version; you may want to give it a try.

Okay, i'll try getting trunk sources from svn and building. I'll reporting back shortly.

The example problem I listed seems to be fixed in trunk. I'll write a few more examples. Thank you FPC development team!

Okay with the latest trunk version I tried compiling my generic classes. It seems there is a problem with fpc generics in that you cannot use a generic as a type parameter. e.g type TDictionary<K,V> =
class(TEnumerable<TPair<K,V>>) ...

Given:

type
  TCollection<T> = class
  end;
  TPair<K,V> = class
    Key: K;
    Value: V;
  end;
// Test
var
  TestA: TCollection<TPair<String,Integer>>; // Fatal: Syntax error, ","
expected but "<" found
  TestB: TCollection<String>; // Okay

The same problem occurs with classes

type
  TDictionaryA<K,V> = class(TCollection<TPair<K,V>>)  // Fatal: Syntax
error, "," expected but "<" found
  end;
  TDictionaryB<K,V> = class(TCollection<Integer>)  // Okay
  end;

Anthony Walter wrote

Okay with the latest trunk version I tried compiling my generic classes. It seems there is a problem with fpc generics in that you cannot use a generic as a type parameter. e.g type TDictionary<K,V> = class(TEnumerable<TPair<K,V>>) ...

Yes, this is a known problem and already on my todo list (like most other generic bugs...)

Regards,
Sven

Thanks Sven. Overall generics in Fpc are very much improved. So good work! Your work is much appreciated!

I also experienced some different problems with classes which implement generic interfaces where the compiler thinks that method aren't implement.

On my blog I have a page where I describe multicast events through delegates source: http://www.codebot.org/delphi/?doc=9568

TDelegateImpl<T> = class(TInterfacedObject, IDelegate<T>)

Fpc gives me some problems with the class above specifically. All the sources are on that page and they currently work with Delphi. I can probably boil the problem into a smaller test case.

If you want some help let me know what I can do.

If you want to report bugs it would be nice if you

1. check whether it was already reported. Most generic bugs have the tag "generics", which you can reach here:
http://bugs.freepascal.org/tag_view_page.php?tag_id=3 (click on "attached issues" in the upper right to see a list of all generics bugs (opened or closed) that are currently in Mantis)
2. add the tag "generics" to new bug reports, so that I can find them quickly.

This would help already if you discover bugs.

Regards,
Sven

Okay Sven I'll be using the bug tracker with the "generics" to assist in the best way I can. I just added an issue for the problem I mentioned which I believe was previously an undocumented issue.

Issue #22019
http://bugs.freepascal.org/view.php?id=22019