Fix assignments of null strings in QML and JavaScript

Assigning "" to a string based property would exhibit inconsistent behavior:

  * When assigned as a literal, it would assign an empty string
  * When assigned from JavaScript, it would assign a null string

When the assignment was done _to_ a QByteArray property, it would hit
the case of calling QVariant::convert where the incoming variant is
either an empty or a null string and the target is a QByteArray. For historical
reasons - as documented - QVariant::convert will return false when the
incoming variant is a null variant.

In V8 assignment from JavaScript would produce an empty string and thus
hit the "succesfull" conversion code path in QVariant to convert to a
seemingly empty QByteArray. With v4 a null string would result in a failed
conversion and spurious warnings as seen in the reported task.

This patch ensures that we consistently map "" to a null string, when it comes
from JavaScript or QML as a literal string. We now also detect the situation of
trying to convert a (valid) null variant to another target type.

Task-number: QTBUG-37197
Change-Id: I68f9031262fdd287d69a38d5468fb38a20441d7b
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>

3 files changed, 34 insertions, 0 deletions

diff --git a/tests/auto/qml/qqmllanguage/data/assignNullStrings.qml b/tests/auto/qml/qqmllanguage/data/assignNullStrings.qml
new file mode 100644
index 0000000000..5e1c3a9b03
--- /dev/null
+++ b/tests/auto/qml/qqmllanguage/data/assignNullStrings.qml
@@ -0,0 +1,9 @@
+import Test 1.0
+MyTypeObject {
+    stringProperty: ""
+    byteArrayProperty: ""
+    function assignNullStringsFromJs() {
+        stringProperty = ""
+        byteArrayProperty = ""
+    }
+}
diff --git a/tests/auto/qml/qqmllanguage/testtypes.h b/tests/auto/qml/qqmllanguage/testtypes.h
index 6a6b2eba45..1c13a2e21c 100644
--- a/tests/auto/qml/qqmllanguage/testtypes.h
+++ b/tests/auto/qml/qqmllanguage/testtypes.h
@@ -236,6 +236,7 @@ class MyTypeObject : public QObject
     Q_PROPERTY(MyMirroredEnum mirroredEnumProperty READ mirroredEnumProperty WRITE setMirroredEnumProperty NOTIFY mirroredEnumPropertyChanged)
     Q_PROPERTY(MyEnumContainer::RelatedEnum relatedEnumProperty READ relatedEnumProperty WRITE setRelatedEnumProperty)
     Q_PROPERTY(QString stringProperty READ stringProperty WRITE setStringProperty NOTIFY stringPropertyChanged)
+    Q_PROPERTY(QByteArray byteArrayProperty READ byteArrayProperty WRITE setByteArrayProperty NOTIFY byteArrayPropertyChanged)
     Q_PROPERTY(uint uintProperty READ uintProperty WRITE setUintProperty NOTIFY uintPropertyChanged)
     Q_PROPERTY(int intProperty READ intProperty WRITE setIntProperty NOTIFY intPropertyChanged)
     Q_PROPERTY(qreal realProperty READ realProperty WRITE setRealProperty NOTIFY realPropertyChanged)
@@ -357,6 +358,15 @@ public:
         emit stringPropertyChanged();
     }
 
+    QByteArray byteArrayPropertyValue;
+    QByteArray byteArrayProperty() const {
+        return byteArrayPropertyValue;
+    }
+    void setByteArrayProperty(const QByteArray &v) {
+        byteArrayPropertyValue = v;
+        emit byteArrayPropertyChanged();
+    }
+
     uint uintPropertyValue;
     uint uintProperty() const {
        return uintPropertyValue;
@@ -570,6 +580,7 @@ signals:
     void qtEnumPropertyChanged();
     void mirroredEnumPropertyChanged();
     void stringPropertyChanged();
+    void byteArrayPropertyChanged();
     void uintPropertyChanged();
     void intPropertyChanged();
     void realPropertyChanged();
diff --git a/tests/auto/qml/qqmllanguage/tst_qqmllanguage.cpp b/tests/auto/qml/qqmllanguage/tst_qqmllanguage.cpp
index 55d07e78da..87f811cbbc 100644
--- a/tests/auto/qml/qqmllanguage/tst_qqmllanguage.cpp
+++ b/tests/auto/qml/qqmllanguage/tst_qqmllanguage.cpp
@@ -110,6 +110,7 @@ private slots:
     void assignLiteralToVariant();
     void assignLiteralToVar();
     void assignLiteralToJSValue();
+    void assignNullStrings();
     void bindJSValueToVar();
     void bindJSValueToVariant();
     void bindJSValueToType();
@@ -873,6 +874,19 @@ void tst_qqmllanguage::assignLiteralToJSValue()
     }
 }
 
+void tst_qqmllanguage::assignNullStrings()
+{
+    QQmlComponent component(&engine, testFileUrl("assignNullStrings.qml"));
+    VERIFY_ERRORS(0);
+    MyTypeObject *object = qobject_cast<MyTypeObject *>(component.create());
+    QVERIFY(object != 0);
+    QVERIFY(object->stringProperty().isNull());
+    QVERIFY(object->byteArrayProperty().isNull());
+    QMetaObject::invokeMethod(object, "assignNullStringsFromJs", Qt::DirectConnection);
+    QVERIFY(object->stringProperty().isNull());
+    QVERIFY(object->byteArrayProperty().isNull());
+}
+
 void tst_qqmllanguage::bindJSValueToVar()
 {
     QQmlComponent component(&engine, testFileUrl("assignLiteralToJSValue.qml"));