The Issue In Brief

Given a QObject derived type defined within a namespace...

namespace MyNamespace {
class MyClass : public QObject
{
...
};
}

If a Q_INVOKABLE function returns a pointer to MyClass to the QmlEngine...

namespace MyNamespace {
class Controller : public QObject
{
    ...

    Q_INVOKABLE MyClass* getMyClass();
}
};

The QmlEngine does not recognize the return type. Console output:
"Error: Unknown method return type: MyClass*"

This occurs even though both MyClass and Controller are properly registered types:

qmlRegisterSingletonType<MyNamespace::Controller>(uri, 1, 0, "Controller", &MyNamespace::Controller::qmlInstance);
    qmlRegisterUncreatableType<MyNamespace::MyClass>(uri, 1, 0, "MyClass", "Get instances via Controller API");

Workaround

I discovered that if I use a fully qualified type name as the return type for the Q_INVOKABLE function, then the issue is avoided.

namespace MyNamespace {
class Controller : public QObject
{
    ...

    Q_INVOKABLE MyNamespace::MyClass* getMyClass();
}
};

I have uploaded a minimal sample plugin and demo app that demonstrates this issue and the workaround.