In the process of upgrading our large hybrid C++/QML1 application from Qt 5.3.2 to 5.4.0, I've noticed that some functionality was broken and that several errors were now logged. Those errors were all a mix of:

"Unable to assign QVariantList to QList<QObject*>"
and
"Error: Cannot assign QVariantList to QList<QObject*>"

After investigation, I found that in 5.4.0, in Qml 1, assigning an empty array to a Q_PROPERTY(QList<QObject*> ...) now fails with the above errors on binding update / assignment.

Thus, given a type "TestObject" with a "prop" QList<QObject*> property, the following Qml snippets now all fail unexpectedly:

TestObject {
  prop: []
}

TestObject {
  id: obj1
  Component.onCompleted: obj1.prop = []
}

TestObject {
  id: obj1
}
TestObject {
  prop: obj1.prop
}

This issue neither occurs on previous Qt or when using Qml 2.

I have found this issue to have been introduced by the following commit in the qtquick1 repository ( see https://codereview.qt-project.org/#/c/91214/ ):

Commit: 9cbb6fab48f10522a012af8e535b1750a553a817 
Author: Robert Griebl <robert.griebl@pelagicore.com>  2014-07-31 10:14:03
Committer: Robert Griebl <robert.griebl@pelagicore.com>  2014-08-05 10:48:36
Parent: dcbd6c64049682fb6cff0e73a5fdd979de7642dd (Merge remote-tracking branch 'origin/5.3' into dev)
Child:  579b773c39c6542b820f037c9a8133dd7c94d1bb (Fix Flickable mouse grab issue.)
Branches: remotes/origin/5.4, remotes/origin/5.4.0, remotes/origin/dev
Follows: v5.3.1
Precedes: v5.4.0-alpha1

    Add handling of value-type lists to QDeclarativeExpression::evaluate()
    
    QDeclarativeExpression::evaluate() converts all arrays to QList<QObject *>,
    even if the items cannot be represented by a QObject *. In case of a
    string-list, a QList of null-pointers is returned (which isn't very helpful).
    This patch makes evaluate() convert arrays, which contain ONLY value-type
    items, into a plain QVariantList.
    
    Change-Id: Ib8452cf9dd0241f146955f5de35336f48b9007c1
    Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>

While this commit introduces desirable behavior of returning arrays of non-QObjects values as pure QVariantLists, it breaks the existing behavior of converting empty arrays as empty QList<QObject*>.

I fixed the issue by modifying the offending change above to return an empty QList<QObject*> when the script value is an array of length 0 and the requested type conversion hint is for QList<QObject*>, as it would do before the change.

I've attached 2 sample applications that demonstrate the issue. The first one (emptyobjectlist) shows a minimal Qml1 application that showcase this issue. The second one (emptyobjectlistsimple) is just standalone app with a main() that shows the issue through QDeclarativeExpressions.