[Development] Switching Qt default builds to C++20 where supported

[Giuseppe D'Angelo]

On 10/10/2024 18:57, Thiago Macieira wrote:
> Re: https://codereview.qt-project.org/c/qt/qtbase/+/463425
> 
> TL;DR: for some std::format functionality, we really want to have out-of-line
> implementations, which requires QtCore compiled with C++20. See
> https://codereview.qt-project.org/c/qt/qtbase/+/595309/2/src/corelib/text/
> qstring.cpp for the thinking. Let's focus first on the C++ language, because
> the ABI discussion is irrelevant if we don't do this.
> 
> So I'd like to propose that we switch all Qt builds to C++20 by default where
> the compilers support it and have supported it for years, but not require it
> for platforms where either the compiler or the standard library are unable to
> compile in C++20 mode.
> 
> This switch can be overridden in the configure/cmake command-line in a
> developer-build-like "use voids warranty" option. That is, the only people who
> should change the default are people reading this mailing list. Other users,
> including all Linux packagers, Homebrew, vcpkg and the official QtC binaries
> should not touch that option. This override is intended solely so we can test
> the ability to compile C++17 without having to install those OSes above (none
> of which are freely available anyway).

Do you foresee any problems at actually guaranteeing that we still work 
with C++17, should one force the C++ version (via whatever means)?
I mean, we have to keep work in C++17 anyhow (for some toolchains), 
what's the harm at saying that Qt actually works everywhere that way?

Is it an implementation concern, or e.g. because by default the CI won't 
test it anymore (will test in C++20), or...?

(Indeed testing is especially tricky. Do we run the full autotest suite 
on those platforms that won't get C++20 by default? Should we still keep 
C++17 coverage around on at least one platform, say, Linux?)


> Moreover, our headers must build with C++17, so this is not a licence to use
> C++20 without compile-time checks (asking for that for the desktop platforms
> is a future discussion).
> 
> Because this is meant to be developer-build-like only, I think we need to
> remove the FEATURE_cxx20 and FEATURE_cxx2b cmake options too. This is because
> some user scripts may be using those and users need to understand the change
> in nature, that this is "voids warranty" now.
> 
> Points of discussion:
> 0) should we do this? Is there any side-effect?
> Please note that downgrading compilers and Standard Library versions has never
> been allowed, so even if the Qt libs get some stickiness to the lib as a
> result of this, that is by itself not a problem. But it would be a problem if
> we do get it on a symbol that is not yet guaranteed to be ABI-stable.

Right now that includes, for instance, libstdc++, which still marks 
C++20 and later as experimental, and can break API/ABI at will.

MS-STL is more conservative, and doesn't expose unstable C++20 features 
behind /std:c++20, only behind c++latest. I'm not sure about libc++.

So, what happens to us if libstdc++ breaks (for instance) ABI in some 
std::format utility?



> 2) how should other modules inherit qtbase's setting?
> The current patch does not inherit the setting at all and, instead, each
> module gets the default again. Each module can be independently overridden
> using CMAKE_CXX_STANDARD.
> 
> I prefer it this way because:
> a) it's simpler, without inter-module dependencies
>    (it was much simpler to implement)
> b) the only people switching the language are us (incl. the CI) and we can
>     simply pass -DCMAKE_CXX_STANDARD where desired
> c) allows for modules to make their own choices if they want to mandate C++17
>    or C++20 for some reason

There's the remote chance that some *private* header may become 
C++20-only, and thus breaking dependent modules, but I wouldn't really 
worry about that...

> d) allows for the CI to test mix-and-match C++17 and C++20 downstream of
>    qtbase
> 
> Alexandru makes a point that some developers may want to have a setting in one
> place and have it inherited to all other modules. This need not remove any of
> the points I made above, though, except that it would be a slightly more
> complex implementation.

What about the qt-configure-module script? Is it going to inherit 
qtbase's settings?


> 3) how should we detect the platforms where C++17 is still required?
> I hardcoded it:
> * QNX, INTEGRITY, VxWorks: C++17 by default
> * everyone else: C++20 by default
> There's no configure-time checking that C++20 works for those platforms where
> it's set by default. The current patch allows CMake to fall back if it can't
> find the functionality, but maybe we just don't allow it to fall back.
> 
> There's an implicit question here whether having a per-platform default is a
> good idea. My stance on this is that I think being held back by platforms that
> won't update more than once a decade puts undue penalty on everyone else who
> does. I don't begrudge anyone from choosing those three above, but I also
> don't want your choices to constrain me.
> 
> 4) should we support building Qt with C++17 on the C++20-by-default platforms?
> Please note this is about building Qt itself, not about user code. User code
> must be compilable with C++17 because we don't know yet whether they have
> content that fails at C++20 for some reason.
> 
> My answer, as shown by my labeling the override option "voids warranty" is a
> very clear and definite NO. This means that if you have GCC 10 or later or
> Clang 13 or thereabouts or later or Visual Studio with the normal Standard
> Libraries, your only supported build is C++20. This means all desktop
> platforms, Android, and Apple mobile devices would use C++20.
> 
> Because we need that build ourselves to check our own headers, we will need to
> keep it working. But there will be consequences, when we do start allowing
> limited ABI dependency (like std::format).

Same question as above: given the C++20 code would still need some sort 
of protection (feature macros, QT_CONFIG, whatever) because it needs to 
build on the C++17 platforms, what's the harm at keeping C++17 "working" 
everywhere?

Effectively, I don't see much difference from the current situation, 
where we "pretend" that we require C++17 but in reality we just check 
that we can pass `-std=c++17` to the compiler. Any usage in Qt code of 
individual language/library features still requires protection, because 
they're not universally implemented.


My 2 c,
-- 
Giuseppe D'Angelo | giuseppe.dangelo at kdab.com | Senior Software Engineer
KDAB (France) S.A.S., a KDAB Group company
Tel. France +33 (0)4 90 84 08 53, http://www.kdab.com
KDAB - Trusted Software Excellence

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