With QT_NO_CAST_FROM_ASCII there is still a set of functions that will take in a const char * and Q_NO_DECLARED_NOT_DEFINED is in qstring.h despite it is not used in Qt 5.x

#include <QString>

int main(int argc, char *argv[])
{
QString str;
str += "test";
return 0;
}

This compiles with the following error on Linux with QT_NO_CAST_FROM_ASCII :

QString& QString::operator+=(const char*) is private within this context

and it shouldn't be just private.