[Development] Qt 5 types under consideration for deprecation / removal in Qt 6

[André Pönitz]

On Sat, Jun 08, 2019 at 06:14:36PM +0200, Giuseppe D'Angelo via Development wrote:
> On 05/06/2019 23:01, André Pönitz wrote:
> > As a matter of fact, some of the previous deprecations, e.g. the removal
> > of qalgorithm, triggered re-implementing the deprecated functionality
> > downstream, effectively shifting the burden of doing (or, rather,
> > *keeping*) them once centrally to all users who need it decentrally.
> 
> The huge impact that these deprecations have is also due to another reason:
> the approach of systematically diverging from "upstream".
> 
> Qt really really fails at not over-extending (in the name of convenience, or
> compatibility, or whatever). Especially for such low-level facilities there
> is a hidden, slow trend at:
> 
> 1) taking something "upstream" lacks, or is available only in very recent
> versions, or is not as widespread as it should be, or similar (limiting
> ourselves to "upstream" == Standard Library, that used to be the case with
> QtAlgorithms and -no-stl; one could say the same with
> QSharedPointer/QWeakPointer, and so on.)
> 
> 2) reimplementing it so that our users have it (yay!)
> 
> 3) giving it easier APIs (yay!)
> 
> 4) *extending* it in incompatible ways with upstream
> (QWeakPointer(QObject*); QSharedPointer in QVariants; etc.).

I see that pattern, too. But now, instead putting the break between 3) and 4),
the whole thing is killed, and everybody downstream has to do 1)-3) again,
or put up with what the standard offers.

And could prevent overextension by -x'ing the respective change on gerrit. 
So *that* sounds fixable.

> Then, it comes a moment when "upstream" stuff has more and more advantages
> -- more speed (algorithms), more flexibility (e.g. mutex classes and
> utilities; shared_ptr<T[]>; etc.), more static analysis tooling, and so on,
> than the equivalent classes offered in Qt.
> 
> In other words, the advantages of keeping the Qt equivalents
> start to be  (seriously) questioned. We're therefore left with the question
> of what to do with these equivalents.
> 
> * We could play the catch-up game, but that requires a development
> investment that is simply not there any more, and is even questionable (is
> it the job of people developing Qt to rewrite algorithms widely available
> elsewhere?).
> 
> * We could move the Qt equivalents into a "support library", maybe with
> deprecation warnings, maybe without. I'm not sure of the traction of this
> idea these days, but IIRC having "Qt4Support" was frowned upon when Qt 5 was
> being shaped. (Thus QtAlgorithms was left in QtCore, deprecated.)
> 
> * We could just deprecate and tell people to migrate away. That's kind of
> the whole point of this thread, and comes with all the annoyances, and
> people reimplementing them downstream because they still want the
> convenience of a qSort(vector) over std::sort(vector.begin(), vector.end()).
> 
> * We could keep things where they are, supported, thus offering the easier
> APIs; but simply reimplement them on top of the "upstream" equivalents.
> (Ignore the possible ABI break.)

The last one is the most reasonable.

> Here's where the "extension" bites us: if the Qt equivalent offered
> something that upstream is not offering, and we can't reimplement it, then
> what do we do? Dropping support for it would be, at best, an API break; and
> at worst, a _silent_ behavioural change.

Proactively preventing a possible future API break by removing things
immediately is a questionable approach. Taken to the extreme this means
each and every bit of Qt should be removed as otherwise it might break API
at some time.

> This is exactly the case of what happened with QtAlgorithms. I believe the
> culprit back then was the usage of qSwap / qLess or so into the algorithms
> themselves (don't remember all the details; also this was pre C++11, and
> using detection idioms and SFINAE to work around this would've been another
> major fun -- file it under "development investment").
> 
> Long story short, making qSort() call std::sort() might not have kept the
> behaviour contract with the user.

*shrug* 

And what if I, as a user, would not care?

What about https://valdyas.org/fading/hacking/happy-porting/ 

   "[...] none, not a single one of all of the reasons you want to deprecate
   Q_FOREACH is a reason I care even a little bit about. It’s going to
   be deprecated? Well, that’s a decision, and a dumb one. It doesn’t
   work on std containers, QVarLengthArray or C arrays? I don’t use
   it on those. It adds 100 bytes of text size? Piffle. It makes it hard
   to reason about the loop for you? I don’t care.

   What I do care is the 1559 places where we use Q_FOREACH in Krita.
   Porting this will take weeks. [...]"

?

 
> Is that break acceptable? Is it not acceptable? It's debatable, as usual;
> but please don't give me the "who in their minds overloads qSwap / qLess /
> qFoobar anyhow?". When it comes to fundamental stuff in QtCore (containers,
> algorithms, etc.) I'd like to stay on the conservative side; and therefore
> the decision, in the end, was to deprecate QtAlgorithms rather than porting
> them over stdlib ones.
> 
> (If you want, it's a corollary of Hyrum's law. The lower you are a software
> stack, the bigger is your number of users => and Hyrum follows...)
> 
> 
> Now, if everyone is unhappy and reimplements QtAlgorithms in their own
> projects, I guess that was the wrong decision? Are we committing it all over
> again when Qt 6 comes and we drop many deprecated APIs? Should we think
> about a Qt5Support module, without API/ABI promises, where to dump things
> without deprecation warnings, just a big bold writing on top "no guarantees
> are given for these things"?

We won't find consensus here.

QT_I_DO_NO_CARE_ABOUT_ABI_BUT_I_CARE_ABOUT_SOURCE_COMPATIBILITY would be a
good starting point for all my personal use cases. For other it isn't.

What I see happening is that projects simply stick for longer with older
versions of Qt the more effort the upgrade is. These are practically 
cold forks, fragmenting the ecosystem. Needlessly, if you ask me.

Andre'