See the below code. (tested 3.2.3 and 3.3.1)

Is this intentionally allowed?

Technically it should work. TSubTest.Test always returns something that fits the inherited requirement.

program Project1;
type
   TFoo = class end;
   TBar = class(TFoo) end;
   TTest = class
     function Test: TFoo; virtual; abstract;
   end;
   TSubTest = class(TTest)
     function Test: TBar; override;
   end;
function TSubTest.Test: TBar;
begin  end;

begin  end.

And if that is intentional, then why not also

   TTest = class
     procedure Test(a: TBar); virtual; abstract;
   end;
   TSubTest = class(TTest)
     procedure Test(a: TFoo); override;
   end;

This fails. Even though any TBar passed will be fine to the base and the subclass.

And any
  var sub: TSubTest
will accept TFoo too, but such a var can never hold the base class.

Martin Frb via fpc-devel schrieb am Sa.,
5. Apr. 2025, 12:02:

Yes, this is intentional and is called "covariant return type" ( https://en.m.wikipedia.org/wiki/Covariant_return_type ).

This is called contra-variance and is in general not something that programming languages support. See for example here: https://stackoverflow.com/questions/2995926/why-is-there-no-parameter-contra-variance-for-overriding

Regards,
Sven

Sven Barth wrote

This is called contra-variance and is in general not something that programming languages support. See for example here: https://stackoverflow.com/questions/2995926/why-is-there-no-parameter-contra-variance-for-overriding

Thanks.

I hadn't planed on using it.

I just run into the return type, wondering why I did not get an error.