After da4a6cf78ff42a4f69c2775997ff174ef647f3f3, moc generates code that is slightly different from the previous, resulting in a different behaviour mode for:

class Outer
{
    class Inner;
};
class Outer::Inner : public QObject
{
    Q_OBJECT
};

This no longer compiles because class Outer::Inner is private and we can't therefore write:

template <> constexpr inline auto qt_call_create_metaobjectdata<TAG>()
{
    return Outer::Inner::qt_create_metaobjectdata<TAG>();
}

Note that meta objects for private classes were not working properly before this change either. The following would fail to compile (and has been failing since 4.0):

class Outer
{
    class Inner;
};
class Outer::Inner : public QObject 
{
    Q_OBJECT
    enum E { V };
    Q_ENUM(E)
};

And the following even fails to moc:

class Outer
{
    class Inner : public QObject {
        Q_OBJECT
    };
};

Output:

standard input:4:1: error: Meta object features not supported for nested classes