Related to: QTBUG-9617
By default Qt is built with QMAKE_CFLAGS =Zc:wchar_t. All fine. But if you want to use QMAKE_CFLAGS = -Zc:wchar_t, then you cannot use fromStdWString and toStdWString.

In Qt4 you used to have

 # ifndef QT_NO_STL_WCHAR
 // workaround for some headers not typedef'ing std::wstring
 typedef std::basic_string<wchar_t> QStdWString;
 # endif // QT_NO_STL_WCHAR

and

 fromStdWDtring, toStdWString overloaded functions:
 static inline QString fromStdWString(const QStdWString &s);
 inline QStdWString toStdWString() const;

In Qt5 those overloaded functions are missing and thus you cannot build without wchar_t as built in type if you need those functions.