Qt configured with "-ltcg" fails to build working apps

I want to build for webassembly, and therefore file size is very important. I recently read https://www.qt.io/blog/reducing-binary-size-of-qt-applications-with-qt-6.8-part-2-coffee-machine-example and though that this should also apply for such builds.

However, when i use my custom configured Qt build, I get linker warnings and the resulting wasm does not run.

Configuration is done by
Qt/6.7.3/wasm_custom$ ../Src/configure -qt-host-path /home/madam/bin/Qt/6.7.3/gcc_64/ -release -optimize-size -ltcg -make libs -no-widgets -platform wasm-emscripten -submodules qtdeclarative -feature-wasm-simd128 -feature-thread -feature-qui
ck-canvas -no-feature-quick-designer -no-feature-quick-particles -no-feature-quick-sprite -no-feature-raster-64bit -no-feature-raster-fp  -prefix /home/madam/bin/Qt/6.7.3/wasm_custom/ && cmake --build . --parallel && cmake --install .

The linker warnings are

em++: warning: -pthread + ALLOW_MEMORY_GROWTH may run non-wasm code slowly, see https://github.com/WebAssembly/design/issues/1271 [-Wpthreads-mem-growth]
cache:INFO: generating system asset: symbol_lists/3ea9a6f15ced4f99606e4cb792e8f80de3cd4e82.json... (this will be cached in "/home/madam/bin/emsdk/upstream/emscripten/cache/symbol_lists/3ea9a6f15ced4f99606e4cb792e8f80de3cd4e82.json" for subsequent builds)
cache:INFO:  - ok
wasm-ld: warning: function signature mismatch: _ZThn8_NK11QWasmScreen10logicalDpiEv
>>> defined as () -> void in /home/madam/bin/Qt/6.7.3/wasm_custom/plugins/platforms/libqwasm.a(qwasmscreen.cpp.o)
>>> defined as (i32, i32) -> void in lto.tmp

wasm-ld: warning: function signature mismatch: _ZNK19QQuickCalendarModel9roleNamesEv
>>> defined as () -> void in /home/madam/bin/Qt/6.7.3/wasm_custom/lib/libQt6QuickTemplates2.a(qquickcalendarmodel.cpp.o)
>>> defined as (i32, i32) -> void in lto.tmp

wasm-ld: warning: function signature mismatch: _ZNK19QAbstractProxyModel22mapSelectionFromSourceERK14QItemSelection
>>> defined as () -> void in /home/madam/bin/Qt/6.7.3/wasm_custom/lib/libQt6Core.a(qabstractproxymodel.cpp.o)
>>> defined as (i32, i32, i32) -> void in lto.tmp

wasm-ld: warning: function signature mismatch: _ZN11QTlsPrivate22X509CertificateGeneric19certificatesFromDerERK10QByteArrayi
>>> defined as () -> void in /home/madam/bin/Qt/6.7.3/wasm_custom/plugins/tls/libqcertonlybackend.a(qx509_generic.cpp.o)
>>> defined as (i32, i32, i32) -> void in lto.tmp

wasm-ld: warning: function signature mismatch: _ZNK19QAbstractProxyModel20mapSelectionToSourceERK14QItemSelection
>>> defined as () -> void in /home/madam/bin/Qt/6.7.3/wasm_custom/lib/libQt6Core.a(qabstractproxymodel.cpp.o)
>>> defined as (i32, i32, i32) -> void in lto.tmp

wasm-ld: warning: function signature mismatch: _ZNK11QTlsPrivate19X509CertificateBase21subjectInfoAttributesEv
>>> defined as () -> void in /home/madam/bin/Qt/6.7.3/wasm_custom/plugins/tls/libqcertonlybackend.a(qx509_base.cpp.o)
>>> defined as (i32, i32) -> void in lto.tmp

wasm-ld: warning: function signature mismatch: _ZNK11QWasmScreen10logicalDpiEv
>>> defined as () -> void in /home/madam/bin/Qt/6.7.3/wasm_custom/plugins/platforms/libqwasm.a(qwasmscreen.cpp.o)
>>> defined as (i32, i32) -> void in lto.tmp

wasm-ld: warning: function signature mismatch: _ZN11QTlsPrivate22X509CertificateGeneric19certificatesFromPemERK10QByteArrayi
>>> defined as () -> void in /home/madam/bin/Qt/6.7.3/wasm_custom/plugins/tls/libqcertonlybackend.a(qx509_generic.cpp.o)
>>> defined as (i32, i32, i32) -> void in lto.tmp

wasm-ld: warning: function signature mismatch: _ZNK11QTlsPrivate19X509CertificateBase20issuerInfoAttributesEv
>>> defined as () -> void in /home/madam/bin/Qt/6.7.3/wasm_custom/plugins/tls/libqcertonlybackend.a(qx509_base.cpp.o)
>>> defined as (i32, i32) -> void in lto.tmp

wasm-ld: warning: function signature mismatch: _ZNK13QQmlListModel9roleNamesEv
>>> defined as () -> void in /home/madam/bin/Qt/6.7.3/wasm_custom/lib/libQt6QmlModels.a(qqmllistmodel.cpp.o)
>>> defined as (i32, i32) -> void in lto.tmp

wasm-ld: warning: function signature mismatch: _ZNK11QTlsBackend17ellipticCurvesIdsEv
>>> defined as () -> void in /home/madam/bin/Qt/6.7.3/wasm_custom/lib/libQt6Network.a(qtlsbackend.cpp.o)
>>> defined as (i32, i32) -> void in lto.tmp

wasm-ld: warning: function signature mismatch: _ZNK11QTlsBackend20systemCaCertificatesEv
>>> defined as () -> void in /home/madam/bin/Qt/6.7.3/wasm_custom/lib/libQt6Network.a(qtlsbackend.cpp.o)
>>> defined as (i32, i32) -> void in lto.tmp

wasm-ld: warning: function signature mismatch: _ZN15QQmlObjectModel7releaseEP7QObjectN17QQmlInstanceModel12ReusableFlagE
>>> defined as () -> void in /home/madam/bin/Qt/6.7.3/wasm_custom/lib/libQt6QmlModels.a(qqmlobjectmodel.cpp.o)
>>> defined as (i32, i32, i32) -> i32 in lto.tmp

wasm-ld: warning: function signature mismatch: _ZN17QQmlDelegateModel7releaseEP7QObjectN17QQmlInstanceModel12ReusableFlagE
>>> defined as () -> void in /home/madam/bin/Qt/6.7.3/wasm_custom/lib/libQt6QmlModels.a(qqmldelegatemodel.cpp.o)
>>> defined as (i32, i32, i32) -> i32 in lto.tmp

wasm-ld: warning: function signature mismatch: _ZNK19QAbstractProxyModel9roleNamesEv
>>> defined as () -> void in /home/madam/bin/Qt/6.7.3/wasm_custom/lib/libQt6Core.a(qabstractproxymodel.cpp.o)
>>> defined as (i32, i32) -> void in lto.tmp

and the runtime error is

RuntimeError: indirect call to null
    _main http://localhost:30000/coffee.js:8
    callMain http://localhost:30000/coffee.js:8
    qtLoad http://localhost:30000/qtloader.js:229
    init http://localhost:30000/coffee.html:48
    onload http://localhost:30000/coffee.html:1
coffee.html:66:25
    init http://localhost:30000/coffee.html:66
    onload http://localhost:30000/coffee.html:1

If binary sizes get reduced by -ltcg, then that would be particularly important for release builds of webassembly. currently, the files are massive, e.g., the coffe example produces a 20mb file. yes, that get's compressed, but it still increases the time to app, as larger files mean longer compilation times in the browser.