After I upgraded KDE Plasma from 5.8.x to 5.9.0, I quickly noticed that the digital clock and taskbar plasmoids stopped working. Hovering over them told me that QtQuick 2.7 is missing. So I grepped over all plasmoids and it revealed that these files: /usr/share/plasma/plasmoids/org.kde.plasma.digitalclock/contents/ui/DigitalClock.qml /usr/share/plasma/plasmoids/org.kde.plasma.taskmanager/contents/ui/ToolTipInstance.qml /usr/share/plasma/plasmoids/org.kde.plasma.taskmanager/contents/ui/ToolTipDelegate.qml contain line: import QtQuick 2.7 So I used in-place sed to change it to: import QtQuick 2.6 and it started working again. I guess that to support both Qt5.6 and 5.7, the QtQuick version specifier should go away completely.
Plasma-5.9 depends on >=Qt-5.7.1 since commit d41718e304c13f90aafa5f58dd3d6aa3d00046b7, it's not just a runtime issue, see also bug 605646.
I've filed an upstream bug - https://bugs.kde.org/show_bug.cgi?id=376199
fixed upstream https://bugs.kde.org/show_bug.cgi?id=376199