Q_UNREACHABLE was reached while compiling some files

Affected 6.7 branch (today's checkout) built from source. Other people report it on 6.8 branch

with qmlcachegen while compiling some QML files, it can hit the following assert:

ASSERT failure in Q_UNREACHABLE(): "Q_UNREACHABLE was reached", file /home/diau/git/qt6/qtdeclarative/src/qmlcompiler/qqmljscodegenerato
r.cpp, line 1407

The culript seems javascript cpnditions written in the form:

    readonly property bool hasVisibleMenu: actionsMenu?.visible ?? false

rewriting in the form

    readonly property bool hasVisibleMenu: actionsMenu ? actionsMenu

won't crash anymore

I's difficult to build a minimal test case as not all files seem to crash, one thing that does always crash is building the KDE's Kirigami repository from cmake