QML to QJsonObject conversion incompatible with JSON.stringify

Hello,

The bug in short is very simple:

consider a QObject "foo" with a 

QVector<QString> bar{"a", "b", "c"};

 property  exposed to QML.

• console.log(JSON.stringify(foo.bar))

    gives 

  [ 'a', 'b', 'c' ]

   as expected.

• passing the object to a C++ slot that takes in argument a QJsonObject from QML, e.g.: 

  backend.toJson(foo.bar)

   gives on the other hand the following JSON structure:

   { "0": "a", "1": "b", "2": "c" }
  

e.g. the array gets transformed into a map, with each index being a key.

This is a somewhat surprising and confusing behaviour. It happens of course recursively, e.g. I'm passing a root object that contains at some point a child, child, child member of this type which gets incorrectly converted.

Attached: full repro