#include <QtWidgets/QApplication>
#include <QtWidgets/QComboBox>
#include <QtWidgets/QCompleter>

int main() {
    QApplication app;
    auto *box = new QComboBox();
    auto *completer = new QCompleter({""});
    box->editTextChanged->connect(completer->complete);
    box->show();
    return app.exec();
}