QFlags: add operator== and !=

It's a value type, and we don't need detours through implicit
conversions (which are a real disgrace, as they make nonsense like
`flags != 3.14` well-formed).

[ChangeLog][QtCore][QFlags] Added overloads of operator== and
operator!= between QFlags objects, and between a QFlags object
and an object of the flag's enumeration.

Change-Id: I36701dea8fcd4cc64941e78af58fa7d38a15a8c7
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qflags.h

@@ -151,6 +151,19 @@ public:
         return on ? (*this |= flag) : (*this &= ~QFlags(flag));
     }
 
+    friend constexpr inline bool operator==(QFlags lhs, QFlags rhs) noexcept
+    { return lhs.i == rhs.i; }
+    friend constexpr inline bool operator!=(QFlags lhs, QFlags rhs) noexcept
+    { return lhs.i != rhs.i; }
+    friend constexpr inline bool operator==(QFlags lhs, Enum rhs) noexcept
+    { return lhs == QFlags(rhs); }
+    friend constexpr inline bool operator!=(QFlags lhs, Enum rhs) noexcept
+    { return lhs != QFlags(rhs); }
+    friend constexpr inline bool operator==(Enum lhs, QFlags rhs) noexcept
+    { return QFlags(lhs) == rhs; }
+    friend constexpr inline bool operator!=(Enum lhs, QFlags rhs) noexcept
+    { return QFlags(lhs) != rhs; }
+
 private:
     constexpr static inline Int initializer_list_helper(typename std::initializer_list<Enum>::const_iterator it,
                                                                typename std::initializer_list<Enum>::const_iterator end)

src/corelib/global/qglobal.cpp

@@ -538,6 +538,30 @@ static_assert(sizeof(qint64) == 8, "Internal error, qint64 is misdefined");
     to seed the calcualtion.
 */
 
+/*!
+    \fn template <typename Enum> bool operator==(QFlags<Enum> lhs, QFlags<Enum> rhs)
+    \fn template <typename Enum> bool operator==(QFlags<Enum> lhs, Enum rhs)
+    \fn template <typename Enum> bool operator==(Enum lhs, QFlags<Enum> rhs)
+    \since 6.2
+    \relates QFlags
+
+    Compares \a lhs and \a rhs for equality; the two arguments are
+    considered equal if they represent exactly the same value
+    (bitmask).
+*/
+
+/*!
+    \fn template <typename Enum> bool operator!=(QFlags<Enum> lhs, QFlags<Enum> rhs)
+    \fn template <typename Enum> bool operator!=(QFlags<Enum> lhs, Enum rhs)
+    \fn template <typename Enum> bool operator!=(Enum lhs, QFlags<Enum> rhs)
+    \since 6.2
+    \relates QFlags
+
+    Compares \a lhs and \a rhs for inequality; the two arguments are
+    considered different if they don't represent exactly the same value
+    (bitmask).
+*/
+
 /*!
   \macro Q_DISABLE_COPY(Class)
   \relates QObject