QMetaProperty::write should reset the property if an empty QVariant is given

[ChangeLog][QtCore][QMetaProperty] write() now resets the property if an
empty QVariant is given, or set a default constructed object if the
property is not resettable

Change-Id: I9f9b57114e740f03ec4db6f223c1e8280a3d5209
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

src/corelib/kernel/qmetaobject.cpp

@@ -3028,6 +3028,11 @@ QVariant QMetaProperty::read(const QObject *object) const
     Writes \a value as the property's value to the given \a object. Returns
     true if the write succeeded; otherwise returns \c false.
 
+    If \a value is not of the same type type as the property, a conversion
+    is attempted. An empty QVariant() is equivalent to a call to reset()
+    if this property is resetable, or setting a default-constructed object
+    otherwise.
+
     \sa read(), reset(), isWritable()
 */
 bool QMetaProperty::write(QObject *object, const QVariant &value) const
@@ -3068,8 +3073,15 @@ bool QMetaProperty::write(QObject *object, const QVariant &value) const
             if (t == QMetaType::UnknownType)
                 return false;
         }
-        if (t != QMetaType::QVariant && int(t) != value.userType() && !v.convert(t))
+        if (t != QMetaType::QVariant && int(t) != value.userType()) {
-            return false;
+            if (!value.isValid()) {
+                if (isResettable())
+                    return reset(object);
+                v = QVariant(t, 0);
+            } else if (!v.convert(t)) {
+                return false;
+            }
+        }
     }
 
     // the status variable is changed by qt_metacall to indicate what it did

src/corelib/kernel/qobject.cpp

@@ -3831,7 +3831,7 @@ int QObjectPrivate::signalIndex(const char *signalName,
   \b{Note:} Dynamic properties starting with "_q_" are reserved for internal
   purposes.
 
-  \sa property(), metaObject(), dynamicPropertyNames()
+  \sa property(), metaObject(), dynamicPropertyNames(), QMetaProperty::write()
 */
 bool QObject::setProperty(const char *name, const QVariant &value)
 {

tests/auto/corelib/kernel/qmetaproperty/tst_qmetaproperty.cpp

@@ -55,7 +55,7 @@ class tst_QMetaProperty : public QObject
     Q_OBJECT
     Q_PROPERTY(EnumType value WRITE setValue READ getValue)
     Q_PROPERTY(EnumType value2 WRITE set_value READ get_value)
-    Q_PROPERTY(QString value7 MEMBER value7)
+    Q_PROPERTY(QString value7 MEMBER value7 RESET resetValue7)
     Q_PROPERTY(int value8 READ value8)
     Q_PROPERTY(int value9 READ value9 CONSTANT)
     Q_PROPERTY(int value10 READ value10 FINAL)
@@ -79,6 +79,7 @@ public:
     void set_value(EnumType) {}
     EnumType get_value() const { return EnumType1; }
 
+    void resetValue7() { value7 = QStringLiteral("reset"); }
     int value8() const { return 1; }
     int value9() const { return 1; }
     int value10() const { return 1; }
@@ -236,6 +237,17 @@ void tst_QMetaProperty::conversion()
     QVERIFY(!customP.write(this, QVariant::fromValue(this)));
     QVERIFY(!value7P.write(this, QVariant::fromValue(this)));
     QVERIFY(!value7P.write(this, QVariant::fromValue<QObject*>(this)));
+
+    // none of this should have changed the values
+    QCOMPARE(value7, hello);
+    QCOMPARE(custom.str, hello);
+
+    // Empty variant should be converted to default object
+    QVERIFY(customP.write(this, QVariant()));
+    QCOMPARE(custom.str, QString());
+    // or reset resetable
+    QVERIFY(value7P.write(this, QVariant()));
+    QCOMPARE(value7, QLatin1Literal("reset"));
 }
 
 QTEST_MAIN(tst_QMetaProperty)

tests/auto/corelib/kernel/qobject/tst_qobject.cpp

@@ -1925,7 +1925,7 @@ void tst_QObject::property()
     QCOMPARE(object.property("string"), QVariant("String1"));
     QVERIFY(object.setProperty("string", "String2"));
     QCOMPARE(object.property("string"), QVariant("String2"));
-    QVERIFY(!object.setProperty("string", QVariant()));
+    QVERIFY(object.setProperty("string", QVariant()));
 
     const int idx = mo->indexOfProperty("variant");
     QVERIFY(idx != -1);
@@ -2027,7 +2027,7 @@ void tst_QObject::property()
     QCOMPARE(object.property("customString"), QVariant("String1"));
     QVERIFY(object.setProperty("customString", "String2"));
     QCOMPARE(object.property("customString"), QVariant("String2"));
-    QVERIFY(!object.setProperty("customString", QVariant()));
+    QVERIFY(object.setProperty("customString", QVariant()));
 }
 
 void tst_QObject::metamethod()