On Tuesday 12 November 2024 04:40:00 Pacific Standard Time Alberto Barbati via
Std-Proposals wrote:
> Hello everyone,
>
> I'd like to know if there's interest in introducing a way to selectively
> ignore the effects of the [[deprecated]] attribute. Such a thing could
> be useful in at least two scenarios:
>
> 1. You have an old API that is being replaced by a new API, but the new
> API is written in terms of the old one (either temporarily or
> because you plan to make the old API private in a future release,
> without completely removing it)
> 2. You have both a “safe” and an “unsafe” API and you want to encourage
> the naïve user to use the safe one, while giving the possibility to
> an experienced user to choose the unsafe API by declaring such
> intent explicitly

3. Unit-testing the deprecated API itself.

This case is the most common occurrence for us by far in Qt. We want to test
this API but at the same time we don't want to use other deprecated API around
it. That means disabling all deprecation warnings is not a good idea. Leaving
them all as warnings is also noisy.

We have a solution for this, which uses pragmas.
https://code.qt.io/cgit/qt/qtbase.git/tree/src/corelib/global/
qcompilerdetection.h?h=v6.8.0#n1180

> 1) Are attributes the right tool?

I think so, because the code is syntactically correct.

> 2) Should we use a different attribute (as "deprecated_ignorable" above)
> or can/should we overload the existing [[deprecated]]?

I don't think so. Either you have a reason to ignore a deprecation or you
don't. The library author has already given you a heads up that this is either
going away or is obsolete for some reason (or actively dangerous).

> 3) The example above uses a string literal to tag the deprecation, so
> that it can be referenced in [[ignore_deprecation]]. Alternatives and
> variations could be considered, including, but not limited to,
> identifiers instead of strings, multiple tags, etc.

I think that is going too far. I'm not opposed to that, but in our experience,
ignoring all deprecations for one statement or block suffices. You can always
make shorter expressions that the ignoring should apply to and that's usually
good code practice. Or use an IILE.

> 4) Should [[ignore_deprecation]] be applied only to compount statements
> (as in the example above) or other statements/entities? Are there scopes
> that could use the attribute that we would be missing (for example
> function default arguments)?

I'd like to see examples of where those would be useful and to see if there
are other solutions to changing the content so it won't need the deprecated
content in there.

For example, if you've deprecated a type, then just don't: deprecate the
constructors instead. If you've deprecated a function that returns the default
value for this function, add a wrapper.

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