Undefined reference errors when using private APIs

I found myself in a situation that called for more control over the text cursor in QML, which was only part of the private API, thus I decided to do a quick and dirty exposing by overriding the class:

// with quick-private in the PRO file
 
#include <QtQuick/private/qquicktextedit_p.h>
#include <QtQuick/private/qquicktextedit_p_p.h>
#include <QtQuick/private/qquicktextcontrol_p.h>

class CTextEdit : public QQuickTextEdit {
    Q_OBJECT
  public:
    CTextEdit(QQuickItem * p = 0) : QQuickTextEdit(p) {}
  public slots:
    void cursorOp(int mode) {
      QQuickTextEditPrivate * ep = reinterpret_cast<QQuickTextEditPrivate *>(d_ptr.data());
      QTextCursor c = ep->control->textCursor();
      c.movePosition((QTextCursor::MoveOperation)mode);
      ep->control->setTextCursor(c);
    }
};

Which turned out to work perfectly, however, there are some problems with getting that code to build. I was getting the following errors:

moc_extra.cpp:-1: error: undefined reference to `QQuickTextControl::textCursor() const'
moc_extra.cpp:-1: error: undefined reference to `QQuickTextControl::setTextCursor(QTextCursor const&)'

Which is not true, because that's a stock Qt class that most certainly implements that functionality.

A user on the Qt forums where I posted this problem informed me that it builds for him, on his build from source.

I eventually managed to get it to build, but only on a custom statically linked Qt build, with literally the same configuration save for the -static option still results in the same undefined reference errors.

I also tried to build that code in an empty project, presuming it may succeed outside of a big and complex project, still no luck.

I also tried the stock Android build, which too failed because of those few lines, but rather than undefined reference errors, I got a:

collect2.exe:-1: error: error: ld returned 1 exit status

Which is definitely caused by those few lines of code, because commenting them out results in a successful build.

So this definitely looks like a bug. Could it have something to do with the deprecation of the declarative module, and its fragmentation into qml and quick?

I did find this bit of documentation, stating that:

All classes that were previously in the QtDeclarative module have been moved into the Qt QML and Qt Quick modules

However, that is either wrong, or applies to a narrow context, for example the classes from the public API.

Nonetheless, even in the Qt builds that fail do build the above code I still have `TextEdit` and the cursor ops definitely work via the keyboard, so that functionality is there, also evident from the fact I did get it to build and work in a static Qt build.

Edit: Being active on SO, I notice other people getting those errors for various private modules too, so it seems it might be a recent issue and a possible regression rather than a problem with a specific class, thus I updated the title to reflect a more general problem.