cmake: GUI build broken with `-stdlib=libc++` #32331

master @ 9a4c92eb9ac29204df3d826f5ae526ab16b8ad65
clang version 20.1.2 (Fedora 20.1.2-3.fc42)
cmake -B build -DBUILD_GUI=ON -DAPPEND_CXXFLAGS="-stdlib=libc++" -DAPPEND_LDFLAGS="-stdlib=libc++"
cmake --build build
<snip>
cmake --build build -j17 --target bitcoin-qt
[  0%] Generating bitcoin-build-info.h
[  0%] Built target secp256k1_precomputed
[  0%] Built target crc32c
[  1%] Built target bitcoin_consensus
[  3%] Built target bitcoin_cli
[  4%] Built target univalue
[  7%] Built target minisketch
[  7%] Built target secp256k1
[ 12%] Built target bitcoin_crypto
[ 20%] Built target leveldb
[ 20%] Built target generate_build_info
[ 20%] Built target bitcoin_clientversion
[ 32%] Built target bitcoin_util
[ 32%] Built target bitcoin_wallet
[ 32%] Built target bitcoinqt_autogen_timestamp_deps
[ 43%] Built target bitcoin_common
[ 44%] Built target bitcoinqt_autogen
[ 63%] Built target bitcoin_node
[100%] Built target bitcoinqt
[100%] Built target bitcoin-qt_autogen_timestamp_deps
[100%] Built target bitcoin-qt_autogen
[100%] Linking CXX executable ../../bin/bitcoin-qt
/usr/bin/ld: CMakeFiles/bitcoin-qt.dir/main.cpp.o: in function `std::__1::__function::__func<$_0, std::__1::allocator<$_0>, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > (char const*)>::operator()(char const*&&)':
/usr/include/x86_64-linux-gnu/qt6/QtCore/qstring.h:1477:(.text+0x18f): undefined reference to `QByteArray::toStdString() const'
/usr/bin/ld: ../../lib/libbitcoinqt.a(bitcoin.cpp.o): in function `QString::toStdString() const':
/usr/include/x86_64-linux-gnu/qt6/QtCore/qstring.h:1477:(.text+0x3b): undefined reference to `QByteArray::toStdString() const'
/usr/bin/ld: /usr/include/x86_64-linux-gnu/qt6/QtCore/qstring.h:1477:(.text+0xe0): undefined reference to `QByteArray::toStdString() const'
/usr/bin/ld: ../../lib/libbitcoinqt.a(bitcoin.cpp.o): in function `BitcoinApplication::createOptionsModel(bool)':
/usr/include/x86_64-linux-gnu/qt6/QtCore/qstring.h:1477:(.text+0xc86): undefined reference to `QByteArray::toStdString() const'
/usr/bin/ld: ../../lib/libbitcoinqt.a(bitcoin.cpp.o): in function `initTranslations(QTranslator&, QTranslator&, QTranslator&, QTranslator&)':
/usr/include/x86_64-linux-gnu/qt6/QtCore/qstring.h:1477:(.text+0x5ab2): undefined reference to `QByteArray::toStdString() const'
/usr/bin/ld: ../../lib/libbitcoinqt.a(bitcoingui.cpp.o):/usr/include/x86_64-linux-gnu/qt6/QtCore/qstring.h:1477: more undefined references to `QByteArray::toStdString() const' follow
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

Is it supposed to work without using depends built with -stdlib=libc++?

IIRC it used to work with qt5, but it is unclear if that was intentional or by accident. I guess in any case this is an upstream issue that would need to be fixed in the distro or in qt6, if it is an issue at all?

Closing for now. I'm not planning on investigating / debugging this any further.