It seems -b option is broken in latest /usr/lib64/qt5/bin/moc. Therefore, it is impossible to build some soft. For example net-im/telegram-desktop::mva. How to reproduce: $ /usr/lib64/qt5/bin/moc -bfoo Unknown option 'bfoo'.
Reported to upstream https://bugreports.qt.io/browse/QTBUG-63706
Gentoo Qt Team, please apply this patch for qtcore-5.9.2 and bump to r1: https://codereview.qt-project.org/208507
Thanks for following up, I'll backport as soon as it lands.
Looks like path is ready.
https://codereview.qt-project.org/#/c/208507/
Fixed in 5.9.3