This conversion causes a lot of pain in cross-platform projects that use both QString and std::wstring. For example, this code compiles fine on Windows but does not on other systems:

std::wstring GetWorld() { return L"world"; }
auto str = QString("Hello %1").arg(GetWorld());

In the past few years I've seen hundreds of failed builds due to this problem alone. It would be nice if Qt had something similar to QT_NO_CAST_FROM_ASCII but for std::wstring.