Generic sub types

[Okoba]

Hello,

I am trying to use a type from another type.
Test is working but Test2 raises an error. Why?

Code: Pascal  [Select][+][-]

1. program Project1;
2.  
3.   {$mode delphi}
4.  
5. type
6.   T1 = object
7.   type
8.     T2 = Integer;
9.   end;
10.  
11.   T3<T> = object
12.     procedure Test;
13.     procedure Test2;
14.   end;
15.  
16.   T4 = T3<T1>;
17.  
18.   procedure T3<T>.Test;
19.   begin
20.     WriteLn(SizeOf(T.T2));
21.   end;
22.  
23.   procedure T3<T>.Test2;
24.   var
25.     A: T.T2; // Error: Error in type definition
26.   begin
27.  
28.   end;
29.  
30. var
31.   V: T4;
32. begin
33.   V.Test;
34.   ReadLn;
35. end.

[ASerge]

I am trying to use a type from another type.
Test is working but Test2 raises an error. Why?

In my opinion, even when compiling the Test procedure, an error should occur. Let 's rewrite it in a different way:

Code: Pascal  [Select][+][-]

1. {$MODE DELPHI}
2. {$APPTYPE CONSOLE}
3.  
4. type
5.   TObj<T> = object
6.     procedure Test;
7.   end;
8.  
9. procedure TObj<T>.Test;
10. begin
11.   WriteLn(SizeOf(T.T2)); // At this point T.T2 is unknown ???
12. end;
13.  
14. type
15.   TClass = class
16.     type T2 = Integer;
17.   end;
18.  
19. var
20.   V: TObj<TClass>;
21. begin
22.   V.Test;
23.   ReadLn;
24. end.

But if we explicitly restrict the template type, then there will be no problems:

Code: Pascal  [Select][+][-]

1. {$MODE DELPHI}
2. {$APPTYPE CONSOLE}
3.  
4. type
5.   TClass = class
6.     type T2 = Integer;
7.   end;
8.  
9.   TObj<T: TClass> = object
10.     procedure Test;
11.   end;
12.  
13. procedure TObj<T>.Test;
14. var
15.   A: T.T2;
16. begin
17.   WriteLn(SizeOf(A));
18. end;
19.  
20. var
21.   V: TObj<TClass>;
22. begin
23.   V.Test;
24.   ReadLn;
25. end.

[PascalDragon]

Test is working but Test2 raises an error. Why?

Please report a bug.

I am trying to use a type from another type.
Test is working but Test2 raises an error. Why?

In my opinion, even when compiling the Test procedure, an error should occur.

FPC's generics are more like C++'s than Delphi's or C#'s so such things do work. Using a class constraint will simply increase type safety (and thus that is indeed the recommended approach anyway).