moc failure when includes inside of enums in namespaces

I'm using Qt 5.13 on macOS together with the MongoCXX driver for MongoDB.

When testing the 5.14 pre-release, moc is not capable handling mongocxx include files.

The build process stops with the following error:

[...] }}{{/opt/qt/5.14/bin/moc -DQT_DEPRECATED_WARNINGS -DQT_CORE_LIB --include /home/pi/MongoMoc/build/moc_predefs.h -I/opt/qt/5.14/mkspecs/linux-g++ -I/home/pi/MongoMoc -I/opt/mongodb/driver/include/mongocxx/v_noabi -I/opt/mongodb/driver/include/bsoncxx/v_noabi -I/opt/mongodb/driver/include/libbson-1.0 -I/opt/mongodb/driver/include/libmongoc-1.0 -I/opt/qt/5.14/include -I/opt/qt/5.14/include/QtCore -I. -I/usr/include/c+/8 -I/usr/include/arm-linux-gnueabihf/c/8 -I/usr/include/c+/8/backward -I/usr/lib/gcc/arm-linux-gnueabihf/8/include -I/usr/local/include -I/usr/lib/gcc/arm-linux-gnueabihf/8/include-fixed -I/usr/include/arm-linux-gnueabihf -I/usr/include ../testobject.h -o moc_testobject.cpp
opt/mongodb/driver-3.3/include/bsoncxx/v_noabi/bsoncxx/types.hp:39: Parse error at "{"[...]

The very same code compiles fine with Qt 5.11.3 on Linux (Raspbian default install) and the Qt 5.13.x series on macOS.

The aforementioned error appears with Qt 5.14-something on Raspbian as well as on macOS Catalina (with the corresponding Xcode).

mongocxx is build from source (https://github.com/mongodb/mongo-cxx-driver.git) and compiled from branch releases/v3.3.

I have attached a minimal (non-)compileable example to this report.