Make qSwap() noexcept, if possible

This greatly increases the value of qSwap(), since not only does it
automatically do the parallel std+ADL lookup of swap(), but also
now centralizes the rather messy code involved to create a correct
noexcept specification.

Other code now can simply use
    Q_DECL_NOEXCEPT_EXPT(noexcept(qSwap(lhs, rhs))).

Change-Id: Ia35df4876b143e86c4150ac452a48c3775c3702b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qglobal.h

@@ -802,8 +802,20 @@ static inline bool qIsNull(float f)
 #  define Q_DUMMY_COMPARISON_OPERATOR(C)
 #endif
 
+namespace QtPrivate
+{
+namespace SwapExceptionTester { // insulate users from the "using std::swap" below
+    using std::swap; // import std::swap
+    template <typename T>
+    void checkSwap(T &t)
+        Q_DECL_NOEXCEPT_EXPR(noexcept(swap(t, t)));
+    // declared, but not implemented (only to be used in unevaluated contexts (noexcept operator))
+}
+} // namespace QtPrivate
+
 template <typename T>
 inline void qSwap(T &value1, T &value2)
+    Q_DECL_NOEXCEPT_EXPR(noexcept(QtPrivate::SwapExceptionTester::checkSwap(value1)))
 {
     using std::swap;
     swap(value1, value2);