[fpc-devel] generic constraints...

[Martin Frb]

On 25/07/2025 15:20, Mattias Gaertner via fpc-devel wrote:
>
>
> On 7/25/25 13:44, Martin Frb via fpc-devel wrote:
>> Currently you can't do
>>
>> type
>>    generic TFoo<A> = class
>>       procedure Bar; virtual;
>>    end;
>>
>>    generic FooChild<F: TFoo> = class(F)
>>       procedure Bar; override;
>>    end;
>
> Can you use:
>
>    generic FooChild<T> = class(specialize TFoo<T>)
>       procedure Bar; override;
>    end;
> ?

No.

One example is that I have several
   TItem1, TItem2, TItem3,...
and for each a
     TItem1List = specialize TFpgList<TItem1>;

But I don't actually use TItem1, ....

I use
   TMyFinalItem1 =
        specialize TFeatureX<
            specialize TFeatureY<
                specialize TFeatureZ< TItem1 >
           >
       >;

And each list needs to be
   TMyFinalItem1 =
        specialize TFeatureXForList<
            specialize TFeatureYForList<
                specialize TFeatureZForList< specialize TFpgList<TItem1>   >
           >
       >;


Currently I can swap the order, and git it working by creating a non 
generic base.... But that introduces dependencies that were not wanted...


Also, the error by the compiler is
      Error: Generics without specialization cannot be used as a type 
for a variable

But
    generic FooChild<F: TFoo> = class(F)

- F is not a variable
- TFoo in not a type (for somethnig), TFoo is a constraint

If that syntax was allowed, it should mean that
-  F must be of any type that is (a descendand of) a specialization of TFoo.

So this is NOT to allow FooChild to be specialized with another generic, 
but with a (any) specialization of that generic.