dev-qt/qtbase-6.9.3 doesn't get on with clang

[Goverp]

For info, I've just posted a bug:
Using clang version 21.1.8 (and I assume earlier versions), kde-misc/kdeconnect-25.08.3-r1 & app-office/kmymoney-5.2.1-r2 won't compile. They produce the following messages in the build log:

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In file included from /usr/include/qt6/QtCore/qdebug.h:41:
/usr/include/qt6/QtCore/qlist.h:449:26: error: constexpr variable 'MaxSize' must be initialized by a constant expression
  449 |         constexpr size_t MaxSize = maxSize()

They compile OK with gcc.

[alamahant]

Its 'flexible' coding in Qt6 Qlist.
If you insist on playing around a bit and not use GCC then you can try

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cp /usr/include/qt6/QtCore/qlist.h /usr/include/qt6/QtCore/qlist.h.bak
grep MaxSize  /usr/include/qt6/QtCore/qlist.h 

Then you can modify

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constexpr size_t MaxSize = maxSize();

to

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static const size_t MaxSize = maxSize();

Emerge the packages and then revert to the original header.
Or create a patch for qlist.h
This is just for fun.
Not a permanent workaround
Clang is behaving a bit bitchy.
Like a mean nanny.
Gcc seems more forgiving...

[GDH-gentoo]

6.9.3 has been dropped from Gentoo's repository, you should probably check if it still happens with current stable, 6.10.1.

Looking at QtBase's current code in its Git repository, I can't see why QList<T>::maxSize() wouldn't be a C++ constant expression for every T.

[Goverp]

6.9.3 has been dropped from Gentoo's repository, you should probably check if it still happens with current stable, 6.10.1.

I wonder if there's some dependency sequencing issue here. I was running the emerge --update yesterday after syncing, so if 6.10.1 is latest, why was (AFAIR) the listing showing the earlier version? I guess it was left from my precious update probably a couple of weeks ago. I'm on the wrong machine to check at the moment - maybe something else is screwed here.

[GDH-gentoo]

So I'm curious, does it happen for 6.10.1? What I see is:

src/corelib/tools/qcontainerfwd.h

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namespace QtPrivate
{
[[maybe_unused]]
constexpr qsizetype MaxAllocSize = (std::numeric_limits<qsizetype>::max)();
}

src/corelib/tools/qarraydata.h

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#include <QtCore/qcontainerfwd.h>
//...
template <class T>
struct QTypedArrayData
    : QArrayData
{
    //...
    constexpr static qsizetype maxSize() noexcept
    {
        // -1 to deal with the pointer one-past-the-end
        return (QtPrivate::MaxAllocSize - sizeof(QtPrivate::AlignedQArrayData) - 1) / sizeof(T);
    }
    //...
};

src/corelib/tools/qarraydataops.h

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#include <QtCore/qarraydata.h>

src/corelib/tools/qarraydatapointer.h

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#include <QtCore/qarraydataops.h>

src/corelib/tools/qlist.h

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#include <QtCore/qarraydatapointer.h>
//...
template <typename T>
class QList
#ifndef Q_QDOC
    : public QListSpecialMethods<T>
#endif
{
    using Data = QTypedArrayData<T>;
    //...
    using DataPointer = QArrayDataPointer<T>;
    //...
    DataPointer d;
public:
    //...
    static constexpr qsizetype maxSize() { return Data::maxSize(); }
    constexpr qsizetype size() const noexcept
    {
        constexpr size_t MaxSize = maxSize();
        Q_PRESUME(size_t(d.size) <= MaxSize);
        return d.size;
    //...
};

And MaxSize's initializer looks like a constant expression to me (QList<T>::maxSize() and QList<T>::Data::maxSize() a.k.a. QTypedArrayData<T>::maxSize() are constexpr member functions, sizeof expressions are constant expressions, and AFAIK QtPrivate::MaxAllocSize is a variable usable in constant expressions)...