[Development] Source break policy for function overloads

[Louai Al-Khanji]

> On Jul 13, 2016, at 9:47 AM, Thiago Macieira <thiago.macieira at intel.com> wrote:
> 
> Em quarta-feira, 13 de julho de 2016, às 10:44:13 PDT, Alexander Blasche 
> escreveu:
>> Hi,
>> 
>> Yesterday, I stumbled over a break in QtSerialBus due to the addition of new
>> QTimer::setInterval(..) overloads in qtbase/dev. This was introduced by
>> 
>> https://codereview.qt-project.org/#/c/160889/
>> 
>> Unfortunately this has the undesirable side effect that lines like:
>> 
>> q->connect(q, &QModbusClient::timeoutChanged, element.timer.data(),
>> &QTimer::setInterval);
>> 
>> do not compile anymore. The function pointer to setInterval() becomes
>> ambiguous. While the fix (https://codereview.qt-project.org/#/c/165034/)
>> was not difficult once you know what is happening this can hit every
>> customer and basically reduces the source compatibility promise into
>> absurdity.
>> 
>> While talking to several devs in the office I could not find a congruent
>> opinion on this issue. Do we or even can we care? Do we have a policy?
> 
> We do have this policy since 5.0 that we have to be careful about overloads 
> due to the connect syntax. The breakage was accidental and not intended.
> 
> I did intend to add overloads, as they're nice to use, I just hadn't 
> considered the consequences.
> 

While I agree that we must be careful when adding new overloads, I think we can do better with QObject::connect as well. I played around with this patch today, it fixes the breakage that sparked this thread without any other modifications:

https://codereview.qt-project.org/165505

There are obviously things to improve in this WIP patch, but I do believe we can make improvements by hinting to the compiler which member function it should choose. I’ll explore this some more unless someone has a clear reason not to go down this road.

Cheers,
Louai
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