Moc behaves weirdly on .cpp files

Running moc manually on a .cpp file produces "note: No relevant classes found. No output generated."

This is visible on examples (e.g. examples/widgets/painting/affine) by running:

~/work/build/qt5/qtbase_moc/bin/moc -I <entries>... xform.cpp

Note: -I entries are taken from the AutogenInfo.json file (see "MOC_INCLUDES") and since automoc seems to work fine, the expectation is that manual moc run should work fine as well.

Similar behavior is seen with QML compiler's autogenerated file (this is how the initial issue was discovered), however, in this case the message is visible even with automoc. At the same time, I can see the "qt_metacall" and friends as symbols of the built binary (so despite "not finding any relevant class", some MOC code is still baked into the binary).