Regression 5.15 to 6.6: Forward Declarations and Q_DECLARE_OPAQUE_POINTER errors

Having huge problems porting mobile business Apps (Android, iOS) using C++ Forward Declarations from 5.15 to 6.6

In my Entity / DTO - data models there are dependencies between QObject* classes in both directions. To avoid cyclic references while importing C++ header, C++ Forward Declaration is used.

This works well in Qt 5.15, but fails in 6.6 with errors like:

Error Pointer Meta Types...must be declared with Q_DECLARE_OPAQUE_POINTER
In file included from moc_MixArticle.cpp:9:
In file included from ./../../_qt_ws6/thielen_eingang_Qt6/cpp/gen/MixArticle.hpp:4:
In file included from ../../_qt_sdks/sdk_6_6b2/6.6.0/android_arm64_v8a/include/QtCore/QObject:1:
In file included from ../../_qt_sdks/sdk_6_6b2/6.6.0/android_arm64_v8a/include/QtCore/qobject.h:18:
../../_qt_sdks/sdk_6_6b2/6.6.0/android_arm64_v8a/include/QtCore/qmetatype.h:1189:13: error: static_assert failed due to requirement 'is_complete<OrderItem, void>::value' "Pointer Meta Types must either point to fully-defined types or be declared with Q_DECLARE_OPAQUE_POINTER(T *)"
            static_assert(is_complete<Pointed, void>::value,
            ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

So I added the recommended Q_DECLARE_OPAQUE_POINTER:

#include <QMetaObject>
...
class OrderItem;
#ifndef OPAQUE_OrderItem
  #define OPAQUE_OrderItem
  Q_DECLARE_OPAQUE_POINTER(OrderItem*)
#endif
...
class MixArticle: public QObject
{
...

see attached file

Yep, in this scenario it is working after adding Q_DECLARE_OPAQUE_POINTER, but not  in all scenarios,

Found out, that it always works for bi-directional references using ForwardDeclaration only at one of the sides as in the example above, where only MixArticle forward-declares OrderItem, but OrderItem includes MixArticle header.

When ForwardDeclarations are used on both sides I'm getting errors.

Then something like this occurred:

error: explicit specialization of 'QtPrivate::IsPointerDeclaredOpaque<MyQObject *>' after instantiation Q_DECLARE_OPAQUE_POINTER(MyQObject*)

Workaround: In my Apps I removed the forward-declaration from one side of all bi-directional references.

This works for most of my 20 mobile business Apps being ported from 5.15 to 6.6

Unfortunately now in very less scenarios C++ now gives me errors like

In included file: main file cannot be included recursively when building a preamble
Serie.hpp:14:10: error occurred here

Nothing complained by Q_DECLARE_OPAQUE_POINTER.

In 5.15 all worked well with forward-declarations on both sides.

The only workaround for now to be able to go on porting my apps: I had to decouple some references to make it compile again. Of course I'm not happy with this: changing my data model and changing C++ and QML code. Have described one of the special cases here at my blog.