QHash compile error when using a key that is a class in a namespace

#include <QCoreApplication>
#include <QHash>
#include <QDebug>

// uncomment to see the compile error
#define USE_FOO_NAMESPACE

#ifdef USE_FOO_NAMESPACE
#define NS foo::
namespace foo {
#else
#define NS
#endif

class Foo {
public:
Foo(int id) : m_id(id) {}

int id() const { return m_id; }

private:
int m_id;
};

#ifdef USE_FOO_NAMESPACE
}
#endif

unsigned int qHash(const NS Foo& foo)
{
return foo.id();
}

bool operator ==(const NS Foo &left, const NS Foo &right)
{
return (left.id() == right.id());
}

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);

NS Foo firstFoo(1);
NS Foo secondFoo(2);

QHash<NS Foo, int> fooHash;
fooHash.insert(firstFoo, 1);
fooHash.insert(secondFoo, 2);

qDebug() << fooHash.count();
}

g++ -c -pipe -g -Wall -W -D_REENTRANT -fPIE -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/mkspecs/linux-g++ -I. -I. -I../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include -I../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtWidgets -I../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtGui -I../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore -I. -o main.o main.cpp
In file included from ../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/qhash.h:1:0,
                 from ../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/QHash:1,
                 from main.cpp:2:
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h: In function ‘uint qHash(const T&, uint) [with T = foo::Foo, uint = unsigned int]’:
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:890:9:   instantiated from ‘QHash<Key, T>::Node** QHash<Key, T>::findNode(const Key&, uint*) const [with Key = foo::Foo, T = int, QHash<Key, T>::Node = QHashNode<foo::Foo, int>, uint = unsigned int]’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:764:36:   instantiated from ‘QHash<Key, T>::iterator QHash<Key, T>::insert(const Key&, const T&) [with Key = foo::Foo, T = int]’
main.cpp:51:27:   instantiated from here
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:111:26: error: no matching function for call to ‘qHash(const foo::Foo&)’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:111:26: note: candidates are:
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:63:13: note: uint qHash(char, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:63:13: note:   no known conversion for argument 1 from ‘const foo::Foo’ to ‘char’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:64:13: note: uint qHash(uchar, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:64:13: note:   no known conversion for argument 1 from ‘const foo::Foo’ to ‘uchar {aka unsigned char}’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:65:13: note: uint qHash(signed char, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:65:13: note:   no known conversion for argument 1 from ‘const foo::Foo’ to ‘signed char’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:66:13: note: uint qHash(ushort, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:66:13: note:   no known conversion for argument 1 from ‘const foo::Foo’ to ‘ushort {aka short unsigned int}’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:67:13: note: uint qHash(short int, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:67:13: note:   no known conversion for argument 1 from ‘const foo::Foo’ to ‘short int’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:68:13: note: uint qHash(uint, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:68:13: note:   no known conversion for argument 1 from ‘const foo::Foo’ to ‘uint {aka unsigned int}’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:69:13: note: uint qHash(int, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:69:13: note:   no known conversion for argument 1 from ‘const foo::Foo’ to ‘int’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:70:13: note: uint qHash(ulong, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:70:13: note:   no known conversion for argument 1 from ‘const foo::Foo’ to ‘ulong {aka long unsigned int}’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:78:13: note: uint qHash(long int, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:78:13: note:   no known conversion for argument 1 from ‘const foo::Foo’ to ‘long int’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:79:13: note: uint qHash(quint64, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:79:13: note:   no known conversion for argument 1 from ‘const foo::Foo’ to ‘quint64 {aka long long unsigned int}’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:87:13: note: uint qHash(qint64, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:87:13: note:   no known conversion for argument 1 from ‘const foo::Foo’ to ‘qint64 {aka long long int}’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:88:13: note: uint qHash(QChar, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:88:13: note:   no known conversion for argument 1 from ‘const foo::Foo’ to ‘QChar’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:89:20: note: uint qHash(const QByteArray&, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:89:20: note:   no known conversion for argument 1 from ‘const foo::Foo’ to ‘const QByteArray&’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:90:20: note: uint qHash(const QString&, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:90:20: note:   no known conversion for argument 1 from ‘const foo::Foo’ to ‘const QString&’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:91:20: note: uint qHash(const QStringRef&, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:91:20: note:   no known conversion for argument 1 from ‘const foo::Foo’ to ‘const QStringRef&’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:92:20: note: uint qHash(const QBitArray&, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:92:20: note:   no known conversion for argument 1 from ‘const foo::Foo’ to ‘const QBitArray&’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:93:20: note: uint qHash(QLatin1String, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:93:20: note:   no known conversion for argument 1 from ‘const foo::Foo’ to ‘QLatin1String’
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:101:32: note: template<class T> uint qHash(const T*, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:109:34: note: template<class T> uint qHash(const T&, uint)
../../../Qt/qt-everywhere-enterprise-src-5.1.1/qtbase/include/QtCore/../../src/corelib/tools/qhash.h:111:29: warning: control reaches end of non-void function [-Wreturn-type]
make: *** [main.o] Error 1