QPair: add a check that SB works as expected

We have no doubt it does, because it's compiler-synthesized, but
we might want to implement the tuple protocol for QPair in the
future and then this will act as a safety net, emulating what
users are currently already doing with QPair.

Change-Id: Ie37f0214bb1aa64210d25be8a256606f4572febe
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

tests/auto/corelib/tools/qpair/tst_qpair.cpp

@@ -36,6 +36,7 @@ class tst_QPair : public QObject
     Q_OBJECT
 private Q_SLOTS:
     void pairOfReferences();
+    void structuredBindings();
     void testConstexpr();
     void testConversions();
     void taskQTBUG_48780_pairContainingCArray();
@@ -121,6 +122,56 @@ void tst_QPair::pairOfReferences()
     QCOMPARE(p.second, QLatin1String("World"));
 }
 
+void tst_QPair::structuredBindings()
+{
+    using PV = QPair<int, QString>;
+    using PR = QPair<int&, const QString&>;
+
+    {
+        PV pv = {42, "Hello"};
+        PR pr = {pv.first, pv.second};
+
+        auto [fv, sv] = pv;
+
+        fv = 24;
+        sv = "World";
+        QCOMPARE(fv, 24);
+        QCOMPARE(sv, "World");
+        QCOMPARE(pv.first, 42);
+        QCOMPARE(pv.second, "Hello");
+
+        auto [fr, sr] = pr;
+
+        fr = 2424;
+        // sr = "World"; // const
+        QCOMPARE(fr, 2424);
+        QCOMPARE(pv.first, 2424);
+    }
+
+    {
+        PV pv = {42, "Hello"};
+        PR pr = {pv.first, pv.second};
+
+        auto& [fv, sv] = pv;
+
+        fv = 24;
+        sv = "World";
+        QCOMPARE(fv, 24);
+        QCOMPARE(sv, "World");
+        QCOMPARE(pv.first, 24);
+        QCOMPARE(pv.second, "World");
+
+        auto& [fr, sr] = pr;
+
+        fr = 4242;
+        //sr = "2World"; // const
+
+        QCOMPARE(fr, 4242);
+        QCOMPARE(pr.first, 4242);
+        QCOMPARE(pv.first, 4242);
+    }
+}
+
 void tst_QPair::testConstexpr()
 {
     Q_CONSTEXPR QPair<int, double> pID = qMakePair(0, 0.0);