Steps to reproduce [Windows]:

• Build qtquickcontrols\examples\quick\controls\gallery
• Run windeployqt [release|debug]
• Run [release|debug]\gallery.exe
• Note error:

  QQmlApplicationEngine failed to load component
  qrc:/main.qml:55 Type MessageDialog unavailable
  qrc:/QtQuick/Dialogs/WidgetMessageDialog.qml:38 module "QtQuick.PrivateWidgets" is not installed
  

• Note: This shows on Windows prominently since windeployqt patches Qt5Core.dll to rely on paths relative to '.', it no longer defaults to the Qt version in path.

Alternatively, compare the logs obtained by:

qmlimportscanner -rootPath . -importPath ...\qtbase\qml