QPair: add member-swap

std::pair has it, too.

Change-Id: I2526b09455db5502ad38a81f3d401098d54614a5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qpair.h

@@ -62,10 +62,22 @@ struct QPair
     { first = std::move(p.first); second = std::move(p.second); return *this; }
 #endif
 
+    Q_DECL_RELAXED_CONSTEXPR void swap(QPair &other)
+        Q_DECL_NOEXCEPT_EXPR(noexcept(qSwap(other.first, other.first)) && noexcept(qSwap(other.second, other.second)))
+    {
+        // use qSwap() to pick up ADL swaps automatically:
+        qSwap(first, other.first);
+        qSwap(second, other.second);
+    }
+
     T1 first;
     T2 second;
 };
 
+template <typename T1, typename T2>
+void swap(QPair<T1, T2> &lhs, QPair<T1, T2> &rhs) Q_DECL_NOEXCEPT_EXPR(noexcept(lhs.swap(rhs)))
+{ lhs.swap(rhs); }
+
 // mark QPair<T1,T2> as complex/movable/primitive depending on the
 // typeinfos of the constituents:
 template<class T1, class T2>

src/corelib/tools/qpair.qdoc

@@ -116,6 +116,31 @@
     \sa qMakePair()
 */
 
+/*!
+    \fn void QPair::swap(QPair &other)
+    \since 5.5
+
+    Swaps this pair with \a other.
+
+    Equivalent to
+    \code
+    qSwap(this->first, other.first);
+    qSwap(this->second, other.second);
+    \endcode
+
+    Swap overloads are found in namespace \c std as well as via
+    argument-dependent lookup (ADL) in \c{T}'s namespace.
+*/
+
+/*!
+    \fn void swap(QPair<T1, T2> &lhs, QPair<T1, T2> &rhs)
+    \overload
+    \relates QPair
+    \since 5.5
+
+    Swaps \a lhs with \a rhs.
+*/
+
 /*! \fn bool operator==(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
 
     \relates QPair