[fpc-devel] generic and type compatibility / interface vs class ...

[Martin Frb]

On 03/02/2026 16:58, Hairy Pixels via fpc-devel wrote:
> On Feb 3, 2026 at 10:42:05 PM, Martin Frb via fpc-devel 
> <fpc-devel at lists.freepascal.org> wrote:
>> https://gitlab.com/freepascal.org/fpc/source/-/issues/41604
>
> program Project1;
> {$Mode objfpc}
> {$Interfaces CORBA}
> type
>    IFoo = interface end;
>    TBar = class(TObject, IFoo) end;
>
>    generic MyGen<A: IFoo> = class end;
>
>    TSome = specialize MyGen<TBar>;
>
> begin
>
> end.
> ok so I can see the whole thing now and this should compile. TBar 
> implements IFoo and "A" must conform to IFoo so TBar is compatible 
> with "A".
>

Not really.

Below stops compiling, when you uncomment the specialize.

IFoo has a method Test.

TBar does not have that. Only TBar as IFoo would have it. But it isn't 
cast to IFoo.



program Project1;
{$Mode objfpc}
{$Interfaces CORBA}
type
   IFoo = interface
     procedure Test;
   end;

   TBar = class(TObject, IFoo)
     procedure BarTest; virtual; abstract;
     procedure IFoo.Test = BarTest;
   end;

   generic MyGen<A: IFoo> = class
     procedure p1(x: A);
   end;

   //TSome = specialize MyGen<TBar>;

procedure MyGen.p1(x: A);
begin
   x.Test
end;

begin

end.
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