On Thursday, 20 March 2025 02:21:29 Pacific Daylight Time Hans Åberg wrote:
> > Too late. Warnings turn into errors, so the linker is never run.
>
> Why do compiler warnings turn into compiler errors?

Because developers compile their code with -Werror or MSVC's -WX to catch
warnings in the build and fix them, as most of the warnings that are printed
are syntactically valid C++, but probably wrong, like type punning or using
the wrong printf placeholder for a given data type. Fixing the warning does
not immediately mean you must change the code: you can simply disable the
warning at that location with a pragma.

But the C++ feature should not rely on developers using pragma, because the
specific pragmas are not part of the standard. The feature must assume that all
warnings will be dealt with as "this must be fixed" and developers will just
change the code.

That means this feature's only remaining use is that an [[unimplemented]]
declaration can later be overridden as implemented. You'll need to explain why
this can't simply be done by changing the first declaration and how that is not
fragile in the first place. What happens if the declaration order changes?

> > [[gnu::weak]] void func();
> > void f()
> > {
> > if (func)
> > func();
> > else
> > fallback();
> >
> > }
> >
> > With LTO or a sufficiently smart linker, it could detect that "func" is
> > indeed defined and suppress the check and the else branch.
> >
> > But this is not possible in all platforms. So it couldn't be part of the
> > C++ standard.
>
> There are two parts: the warning, and the ability to write a temporary
> override.

If it's a temporary thing, I can name my function however I want.

The above is not temporary. We use that technique to write code that works in
both older and newer OSes, wherever supported. That's very common for the
Apple world, where the headers are also smart and react to the -mmacosx-
version-min= command-line option by not emitting the attribute in the first
place.

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
  Principal Engineer - Intel DCAI Platform & System Engineering