Specializing a generic of generic

[Okoba]

Is there anyway I can achieve this with current FPC?
I want to link this types together, so when I specialize TTest3 with for example integer, it I get a TTest2<TTest3<Integer>> in Test2 and TTest1<TTest2<TTest3<Integer>>> in Test1.

Code: Pascal  [Select][+][-]

1. program Project1;
2.  
3.   {$modeSwitch advancedRecords}
4.  
5. type
6.   generic TTest1<T> = record
7.  
8.   end;
9.  
10.   generic TTest2<T> = record
11.   type
12.     TThis = specialize TTest2<T>; //Error: Generics cannot be used as parameters when specializing generics
13.   var
14.     Test1: specialize TTest1<TThis>;
15.   end;
16.   generic TTest3<T> = record
17.   type
18.     TThis = specialize TTest3<T>;
19.   var
20.     Test2: specialize TTest2<TThis>;
21.   end;                                            

[ALLIGATOR]

Everything is OK in Delphi 12

Code: Pascal  [Select][+][-]

1. program test;
2. {$IFDEF FPC}{$mode delphi}{$ENDIF}
3.  
4. type
5.   GFirst<T> = class
6.   type
7.     TThis = GFirst<T>;
8.   end;
9.   GSecond<T> = class
10.   type
11.     TThis = GSecond<T>;
12.     TFirstThis = GFirst<TThis>;
13.   end;
14.  
15. begin
16. end.

[Okoba]

Adding TTest3 creates the problem, please add that and try again.

[ALLIGATOR]

Delphi is fine
In FPC the code above gives an error like yours, but in Delphi everything is OK
But this original code adapted for Delphi compiles successfully in Delphi as well

Code: Pascal  [Select][+][-]

1. program test;
2.  
3. type
4.   TTest1<T> = class
5.   end;
6.  
7.   TTest2<T> = class
8.   type
9.     TThis = TTest2<T>; //Error: Generics cannot be used as parameters when specializing generics
10.   var
11.     Test1: TTest1<TThis>;
12.   end;
13.   TTest3<T> = class
14.   type
15.     TThis = TTest3<T>;
16.   var
17.     Test2: TTest2<TThis>;
18.   end;
19.  
20.   begin
21.   end.

It seems to be a compiler bug

[TRon]

@Okoba

Like this ?

Code: Pascal  [Select][+][-]

1. program test;
2.  
3. {$mode objfpc}{$h+}
4. {$modeSwitch advancedRecords}
5.  
6. type
7.   generic TTest1<T> =
8.   record
9.   end;
10.  
11.   generic TTest2<T> =
12.   record
13.   type
14.     TThis = T;
15.   var
16.     Test1: specialize TTest1<TThis>;
17.   end;
18.   generic TTest3<T> = record
19.   type
20.     TThis = T;
21.   var
22.     Test2: specialize TTest2<TThis>;
23.   end;
24.  
25. begin
26. end.
27.  

[PascalDragon]

Delphi is fine
In FPC the code above gives an error like yours, but in Delphi everything is OK
But this original code adapted for Delphi compiles successfully in Delphi as well

Please note that Okoba's code uses record and not class. In that case the code won't compile in Delphi either.

It seems to be a compiler bug

While there is a bug regarding this, in the end it is very likely that it will not work anyway, because if T in TTest1<> is used as a field the code will fail due to TTest2<> not being completely defined at the point of specialization inside TTest2<>. Generics can not be used to circumvent the need of proper declaration order.

@Okoba

Like this ?

Please compare your code to what Okoba wants. They lead to different behavior (assuming Okoba's code would work).

[Okoba]

@ALLIGATOR Yes it seems.
@TRon No, you are specializing TThis  = T, but I want TThis = TTest3<T>;
@PascalDragon Is there anyway I could archive something like this? I want to have a sub generic type, that calls it's parent. It is done it Test2, but I can not go on with Test3.