Clang-cl + lld-link + Qt 6.11: undefined symbol QConstIterator<QMetaSequence> when iterating QMetaSequence::Iterable

Lan-Friend

Hi,

I am encountering a linking issue with Qt 6.11.0 (msvc2022_64 official build) when using clang-cl and lld-link on Windows.

The problem only appears under a specific combination of conditions involving QSequentialIterable and iterator usage on QMetaSequence::Iterable.

Environment

Qt: 6.11.0 (msvc2022_64 official binary)
Compiler: clang-cl 22.1.7
Linker: lld-link
Build system: CMake + Ninja
C++ standard: C++23
OS: Windows 11

Minimal Context

This code alone works correctly:

#include <QCoreApplication>
#include <QVariant>
#include <QMetaSequence>

void test()
{
    QVariant v = QVariant::fromValue(QList<int>{1, 2, 3});
    auto iterable = v.value<QMetaSequence::Iterable>();

    // No iteration → OK
}

Issue Reproduction

The issue appears when iterator APIs on QMetaSequence::Iterable are used in a translation unit that includes <QSequentialIterable>.

(1) QSequentialIterable is included

#include <QSequentialIterable>

(2) iterator APIs are actually used

For example:

for (auto it = iterable.constBegin(); it != iterable.constEnd(); ++it)
{
    qDebug() << *it;
}

or equivalent begin()/end() or range-based for loops.

Important Clarification

The issue does NOT occur if:

<QSequentialIterable> is included but no iterator API is used

The issue also does NOT occur if:

iterator APIs are used but <QSequentialIterable> is NOT included

It only triggers when:

<QSequentialIterable> + iterator usage on QMetaSequence::Iterable

are combined under clang-cl + lld-link.

Error

lld-link: error: undefined symbol: __declspec(dllimport) public: __cdecl QConstIterator<class QMetaSequence>::QConstIterator<class QMetaSequence>(struct QConstIterator<class QMetaSequence> &&)
>>> referenced by Test\CMakeFiles\Test.dir\Test.cpp.obj:(public: class QTaggedIterator<class QtMetaContainerPrivate::SequentialConstIterator, void> __cdecl QtMetaContainerPrivate::Sequence::constBegin(void) const)
>>> referenced by Test\CMakeFiles\Test.dir\Test.cpp.obj:(public: class QTaggedIterator<class QtMetaContainerPrivate::SequentialConstIterator, void> __cdecl QtMetaContainerPrivate::Sequence::constBegin(void) const)
>>> referenced by Test\CMakeFiles\Test.dir\Test.cpp.obj:(public: class QTaggedIterator<class QtMetaContainerPrivate::SequentialConstIterator, void> __cdecl QtMetaContainerPrivate::Sequence::constEnd(void) const)
>>> referenced 1 more times

Observations

Works correctly with MSVC linker (link.exe)
Fails only with clang-cl + lld-link
Removing <QSequentialIterable> appears to resolve the issue in my environment.
QMetaSequence::Iterable itself works as expected in isolation
The issue is reproducible with iterator APIs only when QSequentialIterable is included

Question

Is this a known ABI / export issue in Qt 6.11 MSVC builds when using clang-cl + lld-link?

Is the combination of <QSequentialIterable> and QMetaSequence::Iterable iterator APIs expected to trigger unresolved symbols in this configuration?

Any clarification or workaround would be appreciated.

Christian Ehrlicher

@Lan-Friend said in Clang-cl + lld-link + Qt 6.11: undefined symbol QConstIterator<QMetaSequence> when iterating QMetaSequence::Iterable:

Is this a known ABI / export issue in Qt 6.11 MSVC builds when using clang-cl + lld-link?

No, but clang-cl is not officially supported: https://doc.qt.io/qt-6/supported-platforms.html#windows
You might create a bug report about it though.