qtbase5-private-dev 5.15.10+dfsg-2 (i386 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 5 applications like Qt Creator QML Designer plugin.
 .
 Use at your own risk.