Qt standard project and examples stopped building

Question

Qt project suddenly stopped building. So as new just created empty projects based on QDialog or examples. Cleaning, rebuilding not helping.

Log of key errors:

/Users/dmitrytolstov/Workspace/Qt521/5.2.1/clang_64/lib/QtWidgets.framework/Versions/5/Headers/qdialog.h:117: 
error: unknown type name 'QDialog'  
Q_DISABLE_COPY(QDialog)

/Users/dmitrytolstov/Workspace/Qt521/5.2.1/clang_64/lib/QtWidgets.framework/Versions/5/Headers/qdialog.h:117: 
error: C++ requires a type specifier for all declarations 
Q_DISABLE_COPY(QDialog)

/Users/dmitrytolstov/Workspace/Qt521/5.2.1/clang_64/lib/QtWidgets.framework/Versions/5/Headers/qdialog.h:117: 
error: unknown type name 'QDialog'

/Users/dmitrytolstov/Workspace/CC++/QtStuff/NewDiaproj/dialog.h:10: 
error: unknown class name 'QDialog'; did you mean 'Dialog'?
class Dialog : public QDialog

/Users/dmitrytolstov/Workspace/CC++/QtStuff/NewDiaproj/dialog.h:10: 
error: base class has incomplete type
class Dialog : public QDialog

/Users/dmitrytolstov/Workspace/CC++/QtStuff/NewDiaproj/main.cpp:8: 
error: no member named 'show' in 'Dialog'
w.show();

7 errors generated.
make: *** [main.o] Error 1
18:46:36: Process «/usr/bin/make» exit with code 2.

Seems like something happened with qdialog.h or something. By the way project on QMainWindow works fine. I didn't do anything. Tried to reopen QtCreator, reboot computer. I use Mac OS X and Qt 5.2.1

Any example provided by QtCreator or empty project based on QDialog. For example: dialog.cpp:

#include "dialog.h"

Dialog::Dialog(QWidget *parent) :
    QDialog(parent)
{
}

Dialog::~Dialog()
{
}

dialog.h

#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>

class Dialog : public QDialog
{
    Q_OBJECT

public:
    explicit Dialog(QWidget *parent = 0);
    ~Dialog();

};

#endif // DIALOG_H

main.cpp

#include "dialog.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Dialog w;
    w.show();

    return a.exec();
}

NewDiaproj.pro

#-------------------------------------------------
#
# Project created by QtCreator 2014-04-20T19:31:45
#
#-------------------------------------------------

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = NewDiaproj
TEMPLATE = app


SOURCES += main.cpp\
        dialog.cpp

HEADERS  += dialog.h

Answer 1

Based on the fact that your files work fine for me on Archlinux with Qt 5.2, I think your QDialog file in the Qt installation got corrupted by some accidental or "vis major" action.

Reinstall it cleanly and then it should just work.