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>

1 files changed, 9 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 = ""
+    }
+}