After upgrading from Qt 5.4.1 to the git of the 5.5 branch (I did that to get access to fixes to QScreen... in XCB QPA to avoid crashes when I switch screens), my KDE Plasma 5.2.1 keeps crashing:

#6 0x00007f6473fa3b85 in isString (this=0x7f6428ffe078) at /var/tmp/portage/dev-qt/qtdeclarative-5.5.9999/work/qtdeclarative-5.5.9999/src/qml/jsruntime/qv4value_inl_p.h:52
#7 QV4::RuntimeHelpers::equalHelper (x=..., y=...) at /var/tmp/portage/dev-qt/qtdeclarative-5.5.9999/work/qtdeclarative-5.5.9999/src/qml/jsruntime/qv4runtime.cpp:708
#8 0x00007f64384db54d in ?? ()
#9 0x00007f6478916700 in ?? ()
#10 0x0001800000000000 in ?? ()
#11 0x0003000000000003 in ?? ()
#12 0x00007f647866c9d0 in ?? ()
#13 0x00007f642a800100 in ?? ()
#14 0x00007f642a800100 in ?? ()
#15 0x00007f64786f7970 in ?? ()
#16 0x00007f6428ffe068 in ?? ()
#17 0x00007fffbef81cd0 in ?? ()
#18 0x00007f6473f447c2 in QV4::SimpleScriptFunction::call (that=<optimized out>, callData=<optimized out>) at /var/tmp/portage/dev-qt/qtdeclarative-5.5.9999/work/qtdeclarative-5.5.9999/src/qml/jsruntime/qv4functionobject.cpp:539
Backtrace stopped: frame did not save the PC

See the attached gdb backtraces and reports from valgrind. `git bisect` identified a first failing commit, e7b66a60584b3fcb1436a7dbe8396b71cb69fe1b ("Make Managed inherit Value").