qt6-base-private-dev 6.4.2+dfsg-18 (amd64 binary) in ubuntu mantic

 Qt is a cross-platform C++ application framework. Qt's primary feature
 is its rich set of widgets that provide standard GUI functionality.
 .
 This package contains the private header development files for building some
 Qt 6 applications like the Qt Creator QML Designer plugin.
 .
 Use at your own risk.