[fpc-devel] Question about generics / what is checked, and what not -- and why?

[Sven Barth]

Martin Frb via fpc-devel <fpc-devel at lists.freepascal.org> schrieb am So.,
19. Okt. 2025, 17:50:

> On 19/10/2025 17:33, Hairy Pixels via fpc-devel wrote:
>
> On Oct 19, 2025 at 10:27:50 PM, Martin Frb via fpc-devel <
> fpc-devel at lists.freepascal.org> wrote:
>
>> Given an non-constrained param, then (in code) any member is assumed to
>> be ok.
>> But in declarations, members of the param can only be used if the
>> compiler knows them.
>>
>>   generic TGenA<T1,T2> = class
>>     X: T1;
>>     Y: T2.unknown;
>>   end;
>>
>> What reason may there be to reject the line for Y?
>>
>
> I agree the compiler can not know that T2.unknown is a valid member before
> specializing so it should be valid syntax.
>
> I think it's just a limitation in the parser. Ideally some subset of
> typeless syntax should be allowed during the initial parse and only only
> validated once the the type is specialized.
>
>
>
> Well, yes, it may be unimplemented in the parser... And maybe that is all
> of it.
>

That's precisely it, nothing more, nothing less. These corner cases are
fixed on a case-by-case base, cause the locations where a fix needs to
happen are easier found that way than trying to understand and fix the
whole parser.

So if you want to see this fixed, then please report a bug.

Regards,
Sven
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